246 research outputs found
Disclinations, dislocations and continuous defects: a reappraisal
Disclinations, first observed in mesomorphic phases, are relevant to a number
of ill-ordered condensed matter media, with continuous symmetries or frustrated
order. They also appear in polycrystals at the edges of grain boundaries. They
are of limited interest in solid single crystals, where, owing to their large
elastic stresses, they mostly appear in close pairs of opposite signs. The
relaxation mechanisms associated with a disclination in its creation, motion,
change of shape, involve an interplay with continuous or quantized dislocations
and/or continuous disclinations. These are attached to the disclinations or are
akin to Nye's dislocation densities, well suited here. The notion of 'extended
Volterra process' takes these relaxation processes into account and covers
different situations where this interplay takes place. These concepts are
illustrated by applications in amorphous solids, mesomorphic phases and
frustrated media in their curved habit space. The powerful topological theory
of line defects only considers defects stable against relaxation processes
compatible with the structure considered. It can be seen as a simplified case
of the approach considered here, well suited for media of high plasticity
or/and complex structures. Topological stability cannot guarantee energetic
stability and sometimes cannot distinguish finer details of structure of
defects.Comment: 72 pages, 36 figure
Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles
publisher: Elsevier articletitle: Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles journaltitle: Carbon articlelink: http://dx.doi.org/10.1016/j.carbon.2017.04.018 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.publisher: Elsevier articletitle: Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles journaltitle: Carbon articlelink: http://dx.doi.org/10.1016/j.carbon.2017.04.018 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.This work was supported in part by the Brazilian Agencies CNPq, CAPES and FAPESP. The authors would like to thank the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID Grant 2013/08293-7. N.M.P. is supported by the European Research Council PoC 2015 “Silkene” No. 693670, by the European Commission H2020 under the Graphene Flagship Core 1 No. 696656 (WP14 “Polymer Nanocomposites”) and under the Fet Proactive “Neurofibres” No. 732344
Morphology of epitaxial core-shell nanowires
We analyze the morphological stability against azimuthal, axial, and general
helical perturbations for epitaxial core-shell nanowires in the growth regimes
limited by either surface diffusion or evaporation-condensation surface
kinetics. For both regimes, we find that geometric parameters (i.e., core
radius and shell thickness) play a central role in determining whether the
nanowire remains cylindrical or its shell breaks up into epitaxial islands
similar to those observed during Stranski-Krastanow growth in thin epilayers.
The combination of small cores and rapid growth of the shell emerge as key
ingredients for stable shell growth. Our results provide an explanation for the
different core-shell morphologies reported in the Si-Ge system experimentally,
and also identify a growth-induced intrinsic mechanism for the formation of
helical nanowires.Comment: In press, Nano Letters (7 pages, 4 figures
New forming method of manufacturing cylindrical parts with nano/ultrafine grained structures by power spinning based on small plastic strains
A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution during the different process stages and macroforming quality of the spun parts made of ASTM 1020 steel are investigated. The results show that the microstructures of the ferrites and pearlites in the ASTM 1020 steel are transformed to the lath martensites after quenching. The martensite laths obtained by quenching are refined to 87 nm and a small amount of nanoscale deformation twins with an average thickness of 20 nm is generated after performing a 3-pass stagger spinning with 55% thinning ratio of wall thickness, where the equivalent strain required is only 0.92. The equiaxial ferritic grains with an average size of 160 nm and nano-carbides are generated by subsequent recrystallization annealing at 480°C for 30 min. The spun parts with high dimensional precision and low surface roughness are obtained by the forming method developed in this work, combining quenching with 3-pass stagger spinning and recrystallization annealing
Thermal Conductivity of Three-Dimensionally Interconnected Graphene-Networked Cu Composite Fabricated by a Simple Two-Step Process
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