Mechanical strength of atomic chains, surface skins, and nanograins

TL;DR

This paper explores how atomic coordination imperfections influence the mechanical strength and thermal stability of nanostructures, revealing the interplay between bond order loss and bond strength gain across different size regimes.

Contribution

It provides a quantitative framework linking atomic bonding characteristics to the thermo-mechanical behavior of low-dimensional systems, including metallic chains and carbon nanotubes.

Findings

Surface is harder at low temperatures but softens near melting point.

Bond order loss reduces melting temperature and dislocation activation energy.

Reproduces inverse Hall-Petch relationship and explains hardening-softening transition.

Abstract

This report deals with the correlation between the mechanical strength and thermal stability of systems extending from monatomic chains to surface skins and solids over the whole range of sizes with emphasis on the significance of atomic coordination imperfection. Derived solutions show that a competition between the bond order loss and the associated bond strength gain of the lower coordinated atoms dictate the thermo-mechanics of the low dimensional systems. Bond order loss lowers the atomic cohesive energy that determines the temperature of melting (Tm), or the activation energy for atomic dislocation, whereas bond strength gain enhances the energy density, or mechanical strength, in the surface skin. Therefore, the surface is harder at T << Tm whereas the surface becomes softer when the T approaches the surface Tm that is lower than the bulk due to bond order loss. Hence, the strained nanostructures are usually stiffer at low T whereas the harder skins melt easier. Quantitative information has been obtained about the bonding identities in metallic monatomic chains and carbon nanotubes. Solutions also enable us to reproduce the inverse Hall-Petch relationship with clarification of factors dominating the transition from hardening to softening in the nanometer regime.