Structural transition in crystalline SiO2 during mechanical amorphization under frictional shear

Abstract

Mechanical milling increases structural disorder in crystalline materials, often resulting in a milling-induced amorphous phase. Prototypical SiO₂ crystals under mechanical shear during milling produce an amorphous phase that has potential for a wide range of applications, from battery anodes to catalyst substrates. Despite its importance, the extent of mechanical amorphization, the mechanochemical reactions, and atomic structure of amorphous materials formed via mechanical deformation remain elusive, because of the lack of experimental probes. Here, multinuclear solid-state nuclear magnetic resonance (NMR) allowed us to uncover the nature of the amorphization of crystalline SiO₂ produced by mechanical milling. The extensive NMR results reveal the milling-induced network depolymerization, hydroxylation, and topological contraction in SiO₂ networks during amorphization. In particular, extreme friction activates chemical interactions among the materials, forming extrinsic chemical bonds via mechanical chemical reactions during deformation under amorphization of SiO₂, indicating that the amorphous phase is not compositionally pure. The current findings with the extrinsic bonds may account for the anomalous increase in the storage capacity of amorphous materials formed by intense milling. The results establish the first quantitative kinetic model of a friction-induced increase in an amorphous SiO₂ complex by a dynamic milling process, identifying the threshold milling rate and duration for the formation of amorphous products. The current results provide predictive and practical guidelines for controlling amorphization through mechanical shear, shedding light on novel synthetic routes to diverse amorphous materials.