A simple numerical approach for reconstructing the atomic stresses at grain boundaries from quantum-mechanical calculations.

The atomistic stress state at a metal grain boundary is an intrinsic attribute which affects many physical and mechanical properties of the metal. While the virial stress is an accepted measure of the atomistic stress in molecular dynamics simulations, an equivalent definition is not well-established for quantum-mechanical density functional theory (DFT) calculations. Here, we introduce a numerical technique, termed the sequential atom removal (SAR) approach, to reconstruct the atomic stresses near a symmetrical-tilt Σ5(310)[001] Cu grain boundary.

Nanofibrillar Si Helices for Low-Stress, High-Capacity Li Anodes with Large Affine Deformations.

We report on the chemical lithiation of long microscale helices composed of densely packed amorphous silicon (aSi) nanofibrils, fabricated by glancing angle deposition (GLAD) through e-beam evaporation. In situ electron microscopy and companion finite element modeling demonstrate that the nanofibrillar structure of the aSi helices allows for 2 orders of magnitude faster effective rates for Li diffusion ( D = 10 cm/s) compared to solid aSi nanowires, while also averting fragmentation during lithiation. More importantly, it is shown that specific helical geometries can accommodate large, lithium-induced, volumetric expansions without shape distortion.

Direct nanomechanical measurements of boron nitride nanotube-ceramic interfaces.

Boron nitride nanotubes (BNNTs) are a unique class of light and strong tubular nanostructure and are highly promising as reinforcing additives in ceramic materials. However, the mechanical strength of BNNT-ceramic interfaces remains largely unexplored. Here we report the first direct measurement of the interfacial strength by pulling out individual BNNTs from silica (silicon dioxide) matrices using in situ electron microscopy techniques.

Interfacial load transfer mechanisms in carbon nanotube-polymer nanocomposites.

Carbon nanotubes (CNTs) are highly promising for strength reinforcement in polymer nanocomposites, but conflicting interfacial properties have been reported by single nanotube pull-out experiments. Here, we report the interfacial load transfer mechanisms during pull-out of CNTs from PMMA matrices, using massively- parallel molecular dynamics simulations. We show that the pull-out forces associated with non-bonded interactions between CNT and PMMA are generally small, and are weakly-dependent on the embedment length of the nanotube.

Nanoscale Mechanics of the Solid Electrolyte Interphase on Lithiated-Silicon Electrodes.

The ∼300% volume changes of lithiated silicon electrodes (LiSi) during electrochemical cycling lead to cracking of the solid electrolyte interface (SEI). Here, we report how strain is transferred from LiSi to two primary inorganic SEI components: LiF and LiO. Our first principle calculations show that LiF, effectively bonded on LiSi at x > 1, enables the entire interface structure to deform plastically by forming delocalized stable voids.

Grain Boundary Traction Signatures: Quantitative Predictors of Dislocation Emission.

We introduce the notion of continuum-equivalent traction fields as local quantitative descriptors of the grain boundary interface. These traction-based descriptors are capable of predicting the critical stresses to trigger dislocation emissions from ductile ⟨110⟩ symmetrical-tilt nickel grain boundaries. We show that Shockley partials are emitted when the grain boundary tractions, in combination with external tensile loading, generate a resolved shear stress to cause dislocation slip.

High damage tolerance of electrochemically lithiated silicon.

Mechanical degradation and resultant capacity fade in high-capacity electrode materials critically hinder their use in high-performance rechargeable batteries. Despite tremendous efforts devoted to the study of the electro-chemo-mechanical behaviours of high-capacity electrode materials, their fracture properties and mechanisms remain largely unknown. Here we report a nanomechanical study on the damage tolerance of electrochemically lithiated silicon.

Brittle-to-ductile transition of lithiated silicon electrodes: Crazing to stable nanopore growth.

Using first principle calculations, we uncover the underlying mechanisms explaining the brittle-to-ductile transition of LixSi electrodes in lithium ion batteries with increasing Li content. We show that plasticity initiates at x = ∼ 0.5 with the formation of a craze-like network of nanopores separated by Si-Si bonds, while subsequent failure is still brittle-like with the breaking of Si-Si bonds.

Communication: Surface-to-bulk diffusion of isolated versus interacting C atoms in Ni(111) and Cu(111) substrates: A first principle investigation.

Using first principle calculations, we study the surface-to-bulk diffusion of C atoms in Ni(111) and Cu(111) substrates, and compare the barrier energies associated with the diffusion of an isolated C atom versus multiple interacting C atoms. We find that the preferential Ni-C bonding over C-C bonding induces a repulsive interaction between C atoms located at diagonal octahedral voids in Ni substrates. This C-C interaction accelerates C atom diffusion in Ni with a reduced barrier energy of ∼1 eV, compared to ∼1.

Atomic-scale mechanisms of sliding along an interdiffused Li-Si-Cu interface.

We perform ab initio calculations on the shear deformation response of the interdiffused Li-Si-Cu phase structure existing between a lithiated Si electrode and a Cu current collector. We show that the formation of well-delineated and weakly bonded Si-Cu and Li-Cu crystalline atomic layers within this phase structure facilitates interface sliding. However, sliding can be terminated by the formation of LiSi3 compounds across these atomic layers, which causes the abrupt capacity fade of the electrode after repeated cycling.