Ductile deformation of core-shell Si-SiC nanoparticles controlled by shell thickness

International audienceAlthough the literature on mechanical properties of nanostructures is extensive, there are still few studies focusing on core-shell nanoparticles. In these systems, which are interesting in a broad range of applications, one could genuinely assume that the softest part, be it the core or the shell, will first yield when submitted to compression. To test this view, we have carried out large scale molecular dynamics simulations of uniaxially compressed core-shell Si-SiC nanoparticles. Our first conclusion is that for the investigated size range (diameters equal or below 50 nm), the nanoparticles yield plastically with no signs of fracture, in agreement with experiments on single material systems. Furthermore, our investigations also reveal that depending on the shell thickness, plastic deformation is confined either in the core or in the shell. We propose a model, based on the theory of contact mechanics and geometrical arguments, to explain this surprising result. Furthermore, we find that for a specific shell to diameter ratio, corresponding to the transition between core and shell, the stress concentration in the nanoparticles is apparently hindered, leading to a delayed plastic deformation

Bimodal distribution of Si-O-Si angles in sodo-silicate glasses

Polarized Raman scattering is performed in a series of sodo-silicate glasses. The Si–O–Si inter-tetrahedral angle and its distribution are extracted using a model developed previously for densified silica, and the result are compared to ab initio atomistic calculations. The two techniques reveal a reduction of the most probable angle of about 0.30°/mol% of Na2O, but the shape of the angular distributions are different. The results suggest that Raman scattering enhances a specific angular distribution of Si–O–Si bridges, likely those close to sodium atoms, highlighting local angular heterogeneities

Uniaxial compression of silicon nanoparticles: An atomistic study on the shape and size effects

International audienceMolecular dynamics simulations were carried out to investigate the mechanical properties of silicon nanoparticles during uniaxial compression by a flat-punch indenter. We considered a large set of systems, with dimensions in the range 10 nm to 50 nm, and various shapes like cubic (perfect and blunt), spherical, truncated spherical, and Wulff-shaped, as well as two compression orientations and two temperatures. Thorough analyses of the simulations first revealed that the relation between nanoparticle size and strength, usually termed as 'smaller is stronger', is critically dependent on the nanoparticle shape, at least for the investigated size range. For instance, a significant and size-dependent strength decrease is determined for facetted Wulff-like nanoparticles, but not for cubic or spherical systems for compression along . We also found that the nanoparticle shape greatly influences plasticity. Several original plasticity mechanisms are obtained, among which the nucleation of half-loop V-shaped dislocation contained in two different {111} planes, dislocations gliding in unusual {110} planes, or the nucleation of partial dislocations in shuffle {111} planes. Our investigations suggest that plasticity properties are mainly governed by the localization of shear stress build up during elastic loading, and the geometry of surfaces in contact with indenters, these two characteristics being intimately related to the nanoparticle shape

The Structure of Sodo-silicate and Alumino-silicate Glasses Observed by Raman Scattering: Experiments and Numerical Simulations

International audienceThe vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The Raman spectra are calculated and the results compare well with the experimental observations. The vibrational analysis confirms the presence of Si-O-Si bending as well as breathing modes of small rings at intermediate frequencies. The Qn Raman-feature at high frequency is decomposed into Q2, Q3, and Q4 vibrational species. Interestingly, the results suggests a large overlap between the three contributions as well as spectral shapes that are not necessarily unimodal nor Gaussian, as frequently assumed in the treatment of the experimental data. Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations (Mg, Ca, Sr, Ba, Na,...) has also been performed. In these glasses, vibrational signatures of cations are clearly evidenced at low frequency in the depolarized (VH) spectra. Raman scattering by cations at low-frequency is confirmed by the computational data in the sodo-silicates. In the alumino-silicate glasses the responses associate to different types of motions. The analysis allows separating the contribution arising from network modifier cations to that originating from charge compensator ones

Mechanical Properties of Amorphous Silicon Nanoparticles

International audienceThe compression of amorphous silicon nanoparticles is investigated by means of molecular dynamics simulations, at two temperatures and for diameters equal to 16 nm and 34 nm. The nanoparticles deform plastically, with maximum contact stresses in the range 8.5-11 GPa, corresponding to strains between 12% and 24%. No clear size effect is observed. Despite large contact stress values, the formation of high density crystalline or amorphous phases is not observed, presumably due to the presence of lateral free surfaces allowing for plasticity deconfinement. Atomic displacements analysis confirms that during plastic deformation, atoms close to indenters are first pushed towards the nanoparticle center, before migrating laterally towards free surfaces. Plastic deformation leads to an increase of fivefold coordinated atoms, which are spatially correlated with the largest atomic displacements