In this study, scanning electronic microscopy, X‐ray diffraction, and thermogravimetric analysis were used in order to study the main characteristics of starch from green‐banana, corn, potato, and cassava, and to analyze the process of thermal degradation of these starches. The Flynn–Wall–Ozawa and modified Coats–Redfern methods were used to determine the activation energy (E) of the degradation process, by using thermogravimetric data. These studies have shown that potato starch exhibited the highest activation energy values during the entire thermal degradation process while corn starch exhibited the lower values. Banana, corn, and potato starches showed that its activation energy was not dependent on the conversion extension, and therefore degradation corresponds to a simple process. In contrast, cassava starch showed that E was dependent of the conversion level, indicating that this degradation was more complex which involved more than one reaction. Potato and banana starches had higher values of the activation energy in its thermal degradation, which could be attributed to greater heterogeneity in granule sizes. Corn starch, which has small and homogeneous granules and major contents of the crystalline fractions, presented a lower activation energy in thermal degradation process.

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Rice husk is considered an important silica source. In this work, microsilica particles were obtained by incineration of rice husk and milling of its ash. The synthesis process was carried out by incinerating the rice husk at 700°C, followed by acid leaching in order to remove inorganic impurities and finally mechanical ball milling at 0, 18, 36 and 72 h to reduce the particle size. Thermogravimetric analysis shows the thermal degradation of rice husk and elimination of organic components. The chemical composition was studied by X-ray fluorescence. The structural properties of silica were determined by X-ray diffraction. The morphological changes caused by mechanical milling were studied by scanning electron microscopy (SEM). The results indicate that between 150 and 450°C, the organic material of the husk was released, and above 550°C, ash with high silica content was obtained. Acid leaching increased the silica purity from 93·40 to 98·22%. According to the X-ray diffraction analysis, the predominant phase was silica. The reduction in the crystallinity of silica samples was shown as an effect of mechanical milling while the particle size was reduced. The percentage of crystallinity decreased from 33·5 to 28·5% for after 72 h of milling. The SEM images showed that the particle size reduced with an increase in milling time, obtaining particle diameters smaller than 2·0 μm. The amorphous silica obtained from rice husk can be used for the production of high-performance silicon, silicon carbide, silicon nitride, silicon tetrachloride and zeolites.

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