The Mechanism of Surface-Radical Generation and Amorphization of Crystalline Quartz Sand upon Mechanochemical Grinding

The structural and morphological changes that occur during the mechanochemical grinding of quartz sand have been characterized by electron paramagnetic resonance (EPR), 29Si solid-state nuclear magnetic resonance (SSNMR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis. BET analysis showed that mechanochemical grinding of quartz sand leads to a significant surface area increase within the first 60 min, which gradually tapers off as the grinding continues (up to 600 min). This rapid initial increase in the surface area does not correlate with mechanoradical and silanol formation; EPR and 29Si SSNMR infer that an initial lag phase exists after which the levels of these species increase substantially. Sequential peak broadening of 29Si SSNMR spectra as well as Si 2p and O 1s XPS core level spectra support matrix amorphization over 600 min of mechanochemical grinding. Amorphization was also observed in the SEM and BET data. These findings suggest that weak points on the surface of quartz sand particles initially fracture, causing a rapid particle size reduction but no significant formation of mechanoradicals. However, once a certain surface area limit is reached, continued grinding leads to abrasion of the freshly fractured surfaces, resulting in the formation of silyl radicals (≡Si•), siloxyl radicals (≡Si–O•), and surface silanols (≡Si–OH and ═Si–(OH)2).