Silica nanofibers with single-digit nanometer-thickness: Origin of unusual viscoelastic behavior

Abstract

Silica has been widely used in functional materials like glasses and protective coatings thanks to its chemical and mechanical stability. Despite its hard and brittle nature, recent molecular dynamics (MD) simulations have revealed unique ductility in silica nanofibers thinner than ~10 nm.1) Our group has synthesized ultrathin silica nanofibers less than 2 nm in diameter, by disassembling a reverse 2D hexagonal mesostructure.2) This presentation focuses on analyzing the nano-mechanics of these nanofibers using MD simulations, particularly their unique viscoelastic properties derived from the kinetics of Si atoms.

Various thicknesses of silica nanofiber models were produced in MD through structural modeling approaches including melt quenching, trimming and surface stabilization. One resulting model of 1.0 nm in diameter, featured an amorphous silica network with a degree of condensation similar to that of the experimentally synthesized nanofiber. Using the Tersoff potential for Si‒O interactions, we applied 0.1 GPa pressure over 500 ps to study the mechanical behavior. Two different thicknesses of silica nanofibers (1.0 and 3.0 nm) depicted different mechanical behavior as shown in Figure 1. The thinner model viscously kept shrunk after unloading, which was well accorded with the SEM image of the synthesized nanofiber. Meanwhile, the thicker nanofiber model indicated the elastic recovery. To understand this viscoelastic transition depending on nanofiber thickness, a percolation analysis was attempted to nanofibers of 1.0 and 1.6 nm in diameter, focusing on the kinetic mobility of Si atoms within the polysiloxane network. In the thicker model, the polysiloxane network contained continuous low kinetic Si atoms, in contrast to the thinner model where connection of low kinetic Si atoms was notably separated with high kinetic Si atoms. Additionally, the thinner model bent at high curvature, which was a characteristic observed in the synthesized nanofiber, particularly around areas with high kinetic Si atoms. These computational analyses complemented experimental findings to provide scientific insight into the unique mechanics of silica nanofibers, which should lay the groundwork for expanding silica applications.

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