10,200 research outputs found
The MRO-accompanied modes of Re-implantation into SiO2-host matrix: XPS and DFT based scenarios
The following scenarios of Re-embedding into SiO2-host by pulsed
Re-implantation were derived and discussed after XPS-and-DFT electronic
structure qualification: (i) low Re-impurity concentration mode -> the
formation of combined substitutional and interstitial impurities with
Re2O7-like atomic and electronic structures in the vicinity of oxygen
vacancies; (ii) high Re-impurity concentration mode -> the fabrication of
interstitial Re-metal clusters with the accompanied formation of ReO2-like
atomic structures and (iii) an intermediate transient mode with Re-impurity
concentration increase, when the precursors of interstitial defect clusters are
appeared and growing in the host-matrix structure occur. An amplification
regime of Re-metal contribution majority to the final Valence Band structure
was found as one of the sequences of intermediate transient mode. It was shown
that most of the qualified and discussed modes were accompanied by the MRO
(middle range ordering) distortions in the initial oxygen subnetwork of the
a-SiO2 host-matrix because of the appeared mixed defect configurations.Comment: 19 pages, 7 figures, accepted to J. Alloys and Compound
Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten
Tungsten is the main candidate material for plasma-facing armour components
in future fusion reactors. In-service, fusion neutron irradiation creates
lattice defects through collision cascades. Helium, injected from plasma,
aggravates damage by increasing defect retention. Both can be mimicked using
helium-ion-implantation. In a recent study on 3000 appm helium-implanted
tungsten (W-3000He), we hypothesized helium-induced irradiation hardening,
followed by softening during deformation. The hypothesis was founded on
observations of large increase in hardness, substantial pile-up and slip-step
formation around nano-indents and Laue diffraction measurements of localised
deformation underlying indents. Here we test this hypothesis by implementing it
in a crystal plasticity finite element (CPFE) formulation, simulating
nano-indentation in W-3000He at 300 K. The model considers thermally-activated
dislocation glide through helium-defect obstacles, whose barrier strength is
derived as a function of defect concentration and morphology. Only one fitting
parameter is used for the simulated helium-implanted tungsten; defect removal
rate. The simulation captures the localised large pile-up remarkably well and
predicts confined fields of lattice distortions and geometrically necessary
dislocation underlying indents which agree quantitatively with previous Laue
measurements. Strain localisation is further confirmed through high resolution
electron backscatter diffraction and transmission electron microscopy
measurements on cross-section lift-outs from centre of nano-indents in
W-3000He
Implanting germanium into graphene
Incorporating heteroatoms into the graphene lattice may be used to tailor its
electronic, mechanical and chemical properties. Direct substitutions have thus
far been limited to incidental Si impurities and P, N and B dopants introduced
using low-energy ion implantation. We present here the heaviest impurity to
date, namely Ge ions implanted into monolayer graphene. Although
sample contamination remains an issue, atomic resolution scanning transmission
electron microscopy imaging and quantitative image simulations show that Ge can
either directly substitute single atoms, bonding to three carbon neighbors in a
buckled out-of-plane configuration, or occupy an in-plane position in a
divacancy. First principles molecular dynamics provides further atomistic
insight into the implantation process, revealing a strong chemical effect that
enables implantation below the graphene displacement threshold energy. Our
results show that heavy atoms can be implanted into the graphene lattice,
pointing a way towards advanced applications such as single-atom catalysis with
graphene as the template.Comment: 20 pages, 5 figure
In situ nanocompression testing of irradiated copper.
Increasing demand for energy and reduction of carbon dioxide emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates a fundamental understanding of how radiation-induced defects alter mechanical properties. Ion beams create radiation damage efficiently without material activation, but their limited penetration depth requires small-scale testing. However, strength measurements of nanoscale irradiated specimens have not been previously performed. Here we show that yield strengths approaching macroscopic values are measured from irradiated ~400 nm-diameter copper specimens. Quantitative in situ nanocompression testing in a transmission electron microscope reveals that the strength of larger samples is controlled by dislocation-irradiation defect interactions, yielding size-independent strengths. Below ~400 nm, size-dependent strength results from dislocation source limitation. This transition length-scale should be universal, but depends on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nanoscale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms
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