Nanoindentation into a bcc high-entropy HfNbTaTiZr alloy-an atomistic study of the effect of short-range order.

The plastic response of the Senkov HfNbTaTiZr high-entropy alloy is explored by means of simulated nanoindentation tests. Both a random alloy and an alloy with chemical short-range order are investigated and compared to the well understood case of an elementary Ta crystal. Strong differences in the dislocation plasticity between the alloys and the elementary Ta crystal are found. The high-entropy alloys show only little relaxation of the indentation dislocation network after indenter retraction and only negligible dislocation emission into the sample interior. Short-range order-besides making the alloy both stiffer and harder-further increases the size of the plastic zone and the dislocation density there. These features are explained by the slow dislocation migration in these alloys. Also, the short-range-ordered alloy features no twinning plasticity in contrast to the random alloy, while elemental Ta exhibits twinning under high stress but detwins considerably under stress relief. The results are in good qualitative agreement with our current knowledge of plasticity in high-entropy alloys.

Spin-lattice-dynamics analysis of magnetic properties of iron under compression.

Compression of a magnetic material leads to a change in its magnetic properties. We examine this effect using spin-lattice dynamics for the special case of bcc-Fe, using both single- and poly-crystalline Fe and a bicontinuous nanofoam structure. We find that during the elastic phase of compression, the magnetization increases due to a higher population of the nearest-neighbor shell of atoms and the resulting higher exchange interaction of neighboring spins. In contrast, in the plastic phase of compression, the magnetization sinks, as defects are created, increasing the disorder and typically decreasing the average atom coordination number. The effects are more pronounced in single crystals than in polycrystals, since the presence of defects in the form of grain boundaries counteracts the increase in magnetization during the elastic phase of compression. Also, the effects are more pronounced at temperatures close to the Curie temperature than at room temperature. In nanofoams, the effect of compression is minor since compression proceeds more by void reduction and filament bending-with negligible effect on magnetization-than by strain within the ligaments. These findings will prove useful for tailoring magnetization under strain by introducing plasticity.

Model based on COVID-19 evidence to predict and improve pandemic control.

Based on the extensive data accumulated during the COVID-19 pandemic, we put forward simple to implement indicators, that should alert authorities and provide early warnings of an impending sanitary crisis. In fact, Testing, Tracing, and Isolation (TTI) in conjunction with disciplined social distancing and vaccination were expected to achieve negligible COVID-19 contagion levels; however, they proved to be insufficient, and their implementation has led to controversial social, economic and ethical challenges. This paper focuses on the development of simple indicators, based on the experience gained by COVID-19 data, which provide a sort of yellow light as to when an epidemic might expand, despite some short term decrements. We show that if case growth is not stopped during the 7 to 14 days after onset, the growth risk increases considerably, and warrants immediate attention. Our model examines not only the COVID contagion propagation speed, but also how it accelerates as a function of time. We identify trends that emerge under the various policies that were applied, as well as their differences among countries. The data for all countries was obtained from ourworldindata.org. Our main conclusion is that if the reduction spread is lost during one, or at most two weeks, urgent measures should be implemented to avoid scenarios in which the epidemic gains strong impetus.

Simulated nanoindentation into single-phase fcc Fe[Formula: see text]Ni[Formula: see text] alloys predicts maximum hardness for equiatomic stoichiometry.

We investigate by molecular dynamics simulation the mechanical behavior of concentrated alloys under nanoindentation for the special example of single-phase fcc Fe[Formula: see text]Ni[Formula: see text] alloys. The indentation hardness is maximum for the equiatomic alloy, [Formula: see text]. This finding is in agreement with experimental results on the strength of these alloys under uniaxial strain. We explain this finding with the increase of the unstable stacking fault energy in the alloys towards [Formula: see text]. With increasing Fe content, loop emission from the plastic zone under the indenter becomes less pronounced and the plastic zone features a larger fraction of screw dislocation segments; simultaneously, the length of the dislocation network and the number of atoms in the stacking faults generated in the plastic zone increase. However, the volume of twinned regions in the plastic zone is highest for the elemental solids and decreases for the alloys. This feature is explained by the fact that twinning proceeds by the glide of dislocations on adjacent parallel lattice planes; this concerted motion is less efficient in the alloys. Finally, we find that surface imprints show increasing pile-up heights with increasing Fe content. The present results will be of interest for hardness engineering or generating hardness profiles in concentrated alloys.

On the mechanical response in nanoalloys: the case of NiCo.

In this work, nanoindentation of spherical NiCo nanoalloys with core-shell and random mixing patterns was studied, and we compared them against monometallic nanoparticles in order to investigate how the mechanical response may be influenced by the elemental distribution and the proportion of each element. Independently of the mixing patterns, plasticity begins with the nucleation of Shockley partial dislocations (SPDs) at the nanoparticle surface, on several slip planes, which leads to the appearance of sessile dislocations and either a stacking fault pyramid (SFP) or an open pyramid at the poles of the spherical nanoalloys. SPDs leave behind stacking faults but, for core-shell structures, the formation of nanotwins was also observed. It was also found that the presence of Co in the external shell of the nanoparticle has the effect of raising the yield strength, which could be interpreted in terms of unstable stacking fault energy. These results have relevance in the design of nanoalloys, since elemental distribution and stoichiometry can be used to tune the desired mechanical properties of the nanoparticle.

Enhancing the Thermal Conductivity of Amorphous Carbon with Nanowires and Nanotubes.

The thermal conductivity of nanostructures can be obtained using atomistic classical Molecular Dynamics (MD) simulations, particularly for semiconductors where there is no significant contribution from electrons to thermal conduction. In this work, we obtain and analyze the thermal conductivity of amorphous carbon (aC) nanowires (NW) with a 2 nm radius and aC nanotubes (NT) with 0.5, 1 and 1.3 nm internal radii and a 2 nm external radius. The behavior of thermal conductivity with internal radii, temperature and density (related to different levels of sp3 hybridization), is compared with experimental results from the literature. Reasonable agreement is found between our modeling results and the experiments for aC films. In addition, in our simulations, the bulk conductivity is lower than the NW conductivity, which in turn is lower than the NT conductivity. NTs thermal conductivity can be tailored as a function of the wall thickness, which surprisingly increases when the wall thickness decreases. While the vibrational density of states (VDOS) is similar for bulk, NW and NT, the elastic modulus is sensitive to the geometrical parameters, which can explain the enhanced thermal conductivity observed for the simulated nanostructures.

Collisions between CO, CO[Formula: see text], H[Formula: see text]O and Ar ice nanoparticles compared by molecular dynamics simulation.

Molecular dynamics simulations are used to study collisions between amorphous ice nanoparticles consisting of CO, CO[Formula: see text], Ar and H[Formula: see text]O. The collisions are always sticking for the nanoparticle size (radius of 20 nm) considered. At higher collision velocities, the merged clusters show strong plastic deformation and material mixing in the collision zone. Collision-induced heating influences the collision outcome. Partial melting of the merged cluster in the collision zone contributes to energy dissipation and deformation. Considerable differences exist-even at comparable collision conditions-between the ices studied here. The number of ejecta emitted during the collision follows the trend in triple-point temperatures and increases exponentially with the NP temperature.

Tension-compression behavior in gold nanoparticle arrays: a molecular dynamics study.

The mechanical properties of Au nanoparticle arrays are studied by tensile and compressive deformation, using large-scale molecular dynamics simulations which include up to 16 million atoms. Our results show that mechanical response is dominated by nanoparticle size. For compression, strength versus particle size shows similar trends in strength than full-density nanocrystals. For diameters (d) below 10 nm there is an inverse Hall-Petch (HP) regime. Beyond a maximum at 10 nm, strength decreases following a HP d -1/2 dependence. In both regimes, interparticle sliding and dislocation activity play a role. The array with 10 nm nanoparticles showed the same mechanical properties than a polycrystalline bulk with the same grain size. This enhanced strength, for a material nearly 20% lighter, is attributed to the absence of grain boundary junctions, and to the array geometry, which leads to constant flow stress by means of densification, nanoparticle rotation, and dislocation activity. For tension, there is something akin to brittle fracture for large grain sizes, with NPs debonding perpendicular to the traction direction. The Johnson-Kendall-Roberts contact theory was successfully applied to describe the superlattice porosity, predicting also the array strength within 10% of molecular dynamics values. Although this study is focused on Au nanoparticles, our findings could be helpful in future studies of similar arrays with NPs of different kinds of materials.

Bouncing window for colliding nanoparticles: Role of dislocation generation.

Available macroscopic theories-such as the Johnson-Kendall-Roberts (JKR) model-predict spherical particles to stick to each other at small collision velocities v; above the bouncing velocity, v_{b}, they bounce. We study the details of the bouncing threshold using molecular dynamics simulation for crystalline nanoparticles where atoms interact via the Lennard-Jones potential. We show that the bouncing velocity strongly depends on the nanoparticle orientation during collision; for some orientations, nanoparticles stick at all velocities. The dependence of bouncing on orientation is caused by energy dissipation during dislocation activity. The bouncing velocity decreases with increasing nanoparticle radius in reasonable agreement with JKR theory. For orientations for which bouncing exists, nanoparticles stick again at a higher velocity, the fusion velocity, v_{f}, such that bouncing only occurs in a finite range of velocities-the bouncing window. The fusion velocity is rather independent of the nanoparticle radius.

Insights into mechanochemical reactions at the molecular level: simulated indentations of aspirin and meloxicam crystals.

Although solvent-free mechanochemical synthesis continues to gain ever greater importance, the molecular scale processes that occur during such reactions remain largely uncharacterised. Here, we apply computational modelling to indentations between particles of crystals of aspirin and meloxicam under a variety of conditions to mimic the early stages of their mechanochemical cocrystallisation reaction. The study also extends to the effects of the presence of small amounts of solvent. It is found that, despite the solid crystalline nature of the reactants and the presence of little or no solvent, mixing occurs readily at the molecular level even during relatively low-energy collisions. When indented crystals are subsequently drawn apart, a connective neck formed by a mixture of the reactant molecules is observed, suggesting plastic-like behaviour of the reacting materials. Overall the work reveals some striking new insights including (i) relatively facile mixing of crystals under solvent-free conditions, (ii) no appreciable local temperature increases, (iii) localised amorphisation at the contact region and neck of the reacting crystals, and (iv) small amounts of solvent have relatively little effect during this early stage of the reaction, suggesting that their accelerating effect on the reaction may be exerted at later stages.

Mechanisms of Iodide⁻Triiodide Exchange Reactions in Ionic Liquids: A Reactive Molecular-Dynamics Exploration.

Efficient charge transport has been observed in iodine-doped, iodide-based room-temperature ionic liquids, yielding high ionic conductivity. To elucidate preferred mechanistic pathways for the iodide ( I - )-to-triiodide ( I 3 - ) exchange reactions, we have performed 10 ns reactive molecular-dynamics calculations in the liquid state for 1-butyl-3-methylimidazolium iodide ([BMIM][I]) at 450 to 750 K. Energy-barrier distributions for the iodine-swapping process were determined as a function of temperature, employing a charge-reassignment scheme drawn in part from electronic-structure calculations. Bond-exchange events were observed with rate-determining energy barriers ranging from ~0.19 to 0.23 ± 0.06 eV at 750 and 450 K, respectively, with an approximately Arrhenius temperature dependence for iodine self-diffusivity and reaction kinetics, although diffusion dominates/limits the bond-exchange events. This charge transfer is not dissimilar in energetics to those in solid-state superionic conductors.

Ion implantation in nanodiamonds: size effect and energy dependence.

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment.

Permanent modifications in silica produced by ion-induced high electronic excitation: experiments and atomistic simulations.

The irradiation of silica with ions of specific energy larger than ~0.1 MeV/u produces very high electronic excitations that induce permanent changes in the physical, chemical and structural properties and give rise to defects (colour centres), responsible for the loss of sample transparency at specific bands. This type of irradiation leads to the generation of nanometer-sized tracks around the ion trajectory. In situ optical reflection measurements during systematic irradiation of silica samples allowed us to monitor the irradiation-induced compaction, whereas ex situ optical absorption measurements provide information on colour centre generation. In order to analyse the results, we have developed and validated an atomistic model able to quantitatively explain the experimental results. Thus, we are able to provide a consistent explanation for the size of the nanotracks, the velocity and thresholding effects for track formation, as well as, the colour centre yield per ion and the colour centre saturation density. In this work we will discuss the different processes involved in the permanent modification of silica: collective atomic motion, bond breaking, pressure-driven atom rearrangement and ultra-fast cooling. Despite the sudden lattice energy rise is the triggering and dominant step, all these processes are important for the final atomic configuration.

The bouncing threshold in silica nanograin collisions.

Using molecular dynamics simulations, we study collisions between amorphous silica nanoparticles. Our silica model contains uncontaminated surfaces, that is, the effect of surface hydroxylation or of adsorbed water layers is excluded. For central collisions, we characterize the boundary between sticking and bouncing collisions as a function of impact velocity and particle size and quantify the coefficient of restitution. We show that the traditional Johnson-Kendall-Roberts (JKR) model provides a valid description of the ingoing trajectory of two grains up to the moment of maximum compression. The distance of closest approach is slightly underestimated by the JKR model, due to the appearance of plasticity in the grains, which shows up in the form of localized shear transformation zones. The JKR model strongly underestimates the contact radius and the collision duration during the outgoing trajectory, evidencing that the breaking of covalent bonds during grain separation is not well described by this model. The adhesive neck formed between the two grains finally collapses while creating narrow filaments joining the grains, which eventually tear.

On the Mechanism of the Iodide-Triiodide Exchange Reaction in a Solid-State Ionic Liquid.

Efficient charge transport has been observed in iodide-based room-temperature ionic liquids when doped with iodine. To investigate preferred pathways for the iodide (I-)-to-triiodide (I3-) exchange reaction and to clarify the origin of this high ionic conductivity, we have conducted electronic structure calculations in the crystal state of 1-butyl-3-methylimidazolium iodide ([BMIM][I]). Energy barriers for the different stages of the iodine-swapping process, including the reorientation of the I-···I3- moiety, were determined from minimum energy paths as a function of a reaction coordinate. Hirshfeld charges and structural parameters, such as bond lengths and angles, were monitored during the reaction. Several bond-exchange events were observed with energy barriers ranging from 0.17 to 0.48 eV and coinciding with the formation of a twisted I-···I3- complex. Striking similarities were observed in the mechanics and energetics of this charge-transfer process in relation to solid-state superionic conductors.

Nucleation of plasticity in nanoparticle collisions.

While at small collision velocities collisions of nanoparticles (NPs) are elastic, they become plastic at higher velocities. We study the elastic-plastic threshold and the onset of plasticity using molecular dynamics simulation for a Lennard-Jones material. The reasons behind the R^{-2/3} increase of the threshold velocity for small NP radii R found recently are discussed. At the threshold, NP orientation strongly influences the generation of plasticity, and averaging over many orientations is required to predict the critical velocity for dislocation generation. The onset of plasticity is governed by the generation of isolated stacking faults and nanotwins spanning the entire NP. At higher velocities, the fraction of defects becomes proportional to the total number of atoms in the NP.

The elastic-plastic transition in nanoparticle collisions.

When nanoparticles (NPs) collide with low velocities, they interact elastically in the sense that--besides their fusion caused by their mutual van-der-Waals attraction--no defects are generated. We investigate the minimum velocity, vc, necessary for generating defects and inducing plasticity in the NP. The determination of this elastic-plastic threshold is of prime importance for modeling the behavior of granular matter. Using the generic Lennard-Jones interaction potential, we find vc to increase strongly with decreasing radius. Current models do not agree with our simulations, but we provide a model based on dislocation emission in the contact zone that quantitatively describes the size dependence of the elastic-plastic transition.

Compaction of highly porous granular matter by impacts on a hard wall.

Using a granular-mechanics code, we study the impact of a highly porous granular body on a hard wall. The projectile consists of monodisperse adhesive micrometer-sized silica grains. For the impact velocities studied, v<0.5m/s, the sample does not fragment, but is compacted. We find that the compaction is proportional to the impact speed. The proportionality constant increases with decreasing porosity. However, the compaction is inhomogeneous and decreases with distance from the target. A compaction wave runs through the aggregate; it slows down while the compaction becomes less efficient.

Analytical solution of the mean field Ising model for finite systems.

The Ising model for finite systems, e.g. for clusters of different sizes and crystal lattices, was solved analytically by the mean field approach. The magnetization was calculated from the number of accessible microstates, using the gamma function and its derivatives, unlike the usual solution in the microcanonical which uses the Stirling approximation. We determined a scaling exponent of ∼1/3, which shows how the Curie temperature decreases with decreasing nanoparticle size. Moreover, the model predicts the behaviour of surface and core regions and it explains in simple terms several effects previously observed in experiments and Monte Carlo simulations of small magnetic systems.

Why nanoprojectiles work differently than macroimpactors: the role of plastic flow.

Atomistic simulation data on crater formation due to the hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to 1.1×10(10) atoms are compared to available experimental data on µm-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nanoregime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as is expected in larger microscale experiments. The importance of the strain-rate dependence of strength and of dislocation production and motion are discussed.