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Results for “pyramidal defect inversion”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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2 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Electronically Nonadiabatic Structural Transformations Promoted by Electron Beams

The use of scanning transmission electron microscopes to manipulate substitutional defects in graphene has recently been demonstrated and modeled using ground state molecular dynamics, but the role of electronic excitations induced through inelastic electron scattering in promoting these transformations has so-far remained unexplored. In this work, probed are the effects of electronic excitation on the structural dynamics of graphene quantum dots of differing edge morphologies that are substitutionally doped with silicon or phosphorous. The ground and excited state potential energy barriers for pyramidal inversion of these nonplanar doped species are evaluated using time-dependent density functional theory. Optically bright excited states in which the barrier is decreased are identified in the low energy region of the electronic spectrum, suggesting that photoexcitation can modulate the reactivity of defects in graphene under electron beam irradiation. Coupling matrix elements between these inversion-favoring excited states and the ground state and time-domain simulations of the material's response to a point charge impulse indicate that focusing an electron beam near the defect can also lead to population of these states, implying that beam electrons incident on a defect can both excite the material to an inversion-favoring state and transfer momentum to the defect to initiate the inversion.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Asymmetric equilibrium core structures of pyramidal-II < c + a > dislocations in ten hexagonal-close-packed metals

The structures of pyramidal-II < c + a > dislocations, one of the most important defects in structural hexagonal-close-packed (HCP) metals, have not been fully characterized for many of the HCP metals in use today. Here, we employ ab initio informed phase-field dislocation dynamics to determine the minimum energy structure of pyramidal {1¯1¯22} < 11¯23 > dislocations in ten HCP metals, including Be, Co, Mg, Re, Ti, Zn, Cd, Hf, Y, and Zr. As input for the simulations, we calculate, using first-principles density functional theory, the {1¯1¯22} generalized stacking fault energy (GSFE) curves for all ten metals. From these calculations, it is found that magnetism in Co is necessary for achieving a local minimum in the GSFE curve. We observe in simulations that edge and screw character dislocations split into two partials separated by a low-energy intrinsic stacking fault. The splitting distance is shown to scale inversely with the local minimum energy normalized by the product of its shear modulus and Burgers vector. Interestingly, some HCP metals exhibit an asymmetric structure, with either unequal partial Burgers vectors or widths, in contrast to the symmetric configuration expected from linear elastic dislocation theory. We explain these structures by properties of the local maxima in their GSFE curves. Metals with larger degrees of elastic anisotropy result in dislocations with larger splitting distances than would be expected under the commonly used assumption of elastic isotropy. Furthermore, these findings on the sizes and asymmetry in the structures of pyramidal-II < c + a > dislocations are fundamental to understanding how these dislocations glide and interact or react with other defects when these metals are mechanically strained.

36 MATERIALS SCIENCE↗