Mechanics of Nano-Honeycomb Silica Structures: Size-Dependent Brittle-to-Ductile Transition

Abstract

Porous silica structures with intricate design patterns form the exoskeleton of diatoms, a large class of microscopic mineralized algae, whose structural features have been observed to exist down to nanoscale dimensions. Nanoscale patterned porous silica structures have also been manufactured for the use in optical systems, catalysts, and semiconductor nanolithography. The mechanical properties of these porous structures at the nanoscale are a subject of great interest for potential technological and biomimetic applications in the context of new classes of multifunctional materials. Previous studies have established the emergence of enhanced toughness and ductility in nanoporous crystalline silica structures over bulk silica. The authors undertake molecular dynamics simulations and theoretical size-scaling studies of elasticity and strength of a simple model of generic nanoporous silica structures, used to establish a theoretical model for the detailed mechanisms behind their improved properties, and show through theoretical analysis that below a critical length scale around 60–80 Å, the silica struts in the nanoporous structure undergo plastic shear deformation before fracture, leading to enhanced ductility. Corresponding molecular dynamics simulations directly confirm that at this critical length scale, the fracture mechanism changes from crack propagation starting at regions of stress concentration to plasticity showing shearing and necking. This drastic change in the material behavior arises from the flaw-tolerant size of the constituent silica struts, where below a critical width, the strut fails at the theoretical shear strength of silica. The insight developed from these theoretical-computational studies could be used to engineer silica structures that deform plastically before fracture and are much tougher than bulk silica. Such structures have potential application in carrying load, along with catalytic, optical, and adsorbent applications for novel nanodevices and nanomaterials.