Time-Scaling in Atomistics and the Mechanical Behavior of Materials




Journal Title

Journal ISSN

Volume Title



Modeling physical phenomena with atomistic fidelity and at laboratory time-scales is one of the holy grails of computational materials science. Conventional molecular dynamics (MD) simulations enable the elucidation of an astonishing array of phenomena inherent in the mechanical and chemical behavior of materials. However, conventional MD, with our current computational modalities, is incapable of resolving time-scales longer than microseconds (at best).

In this dissertation, using a recently proposed approach---the so-called autonomous basin climbing (ABC) method--- together with other techniques, including nudged elastic band (NEB), kinetic Monte Carlo (KMC), and transition state theory (TST), we provide several insights on some key problems in material science. The following topics are addressed:

(i) Li diffusion in amorphous matrix: Using ABC-based approach, a realistic evaluation of Li-ion diffusion pathways in amorphous Si is evaluated. Diffusive pathways are not a priori set, but rather emerge naturally as part of our computation. The comparative differences between Li-ion diffusion in amorphous and crystalline Si is elucidated.

(ii) Rate-dependent mechanical behavior of crystalline nano-structure: A study of the mechanical compression behavior of nano-slabs to specifically interrogate its deformation behavior under both slow and fast strain rates. While high-strain rate deformation proceeds in an unremarkable manner---merely shortening its length along with the formation of an expected defect sub-structure, the slow-strain rate results (---precisely what is to be expected in most applications and laboratory experiments) exhibit a dramatically different behavior. We observe "liquid-like" deformation under low strain rate.

(iii) Elucidating the micro-mechanisms of rate-dependent plasticity in a-LiSi nano-structure: Silicon is arguably one of the most important electrode materials in Li-battery system. In this study, we conduct slow strain rate computational studies to provide insights into the mechanisms underpinning plastic deformation of amorphous Li-Si. We find that in the case of Li-Si nano-structures, the basic mechanism of plasticity are similar to what has been discussed in other amorphous system---formation of shear transformation zone engineered by diffusion like process. We also identify the rotation of the STZ as a key dissipation mechanism. Furthermore, the behavior under high & low strain rate is quite different and accordingly convectional MD cannot be used to understand plasticity.



Time-scaling atomistic simulation, Autonomous basin climbing, Li-Si alloy, Nanostructures


Portions of this document appear in: Yan, Xin, Afif Gouissem, and Pradeep Sharma. "Atomistic insights into Li-ion diffusion in amorphous silicon." Mechanics of Materials 91 (2015): 306-312. And in: Yan, Xin, and Pradeep Sharma. "Time-scaling in atomistics and the rate-dependent mechanical behavior of nanostructures." Nano letters 16, no. 6 (2016): 3487-3492.