Ejection of Glycine Molecules Adsorbed on a Water Ice Surface by Swift-heavy Ion Irradiation

Organic molecules may be adsorbed on the ice surfaces of comets or moons. We study the desorption process induced by swift-heavy ion irradiation using a molecular dynamics simulation. Focusing on the amino acid glycine adsorbed on water ice as a prototypical example, we model a 2 MeV sulfur ion impact as it might be typical of magnetospheric ion impact on the surface of Europa. We find that molecules are ejected intact within a radius of up to 25 Å around the ion impact point. Within a core region of around 10 Å, glycine molecules are destroyed and mainly fragments are emitted. Prominent fragments produced are cyanide CN-, carbon monoxide CO, cyanate OCN-, and carbon dioxide CO2, in agreement with experimental studies. In addition, radiolysis of water ice generates the radicals H+, H3O+, and HO- as well as the gases H2, O2, and some H2O2. While the smaller fragments easily obtain velocities above 2 km s-1 - the escape velocity from Europa - most ejected glycine molecules obtain smaller velocities and will thus not leave the moon permanently. Our results thus provide a detailed example that shows to what extent intact emission of organic molecules from Europa's surface by ion irradiation is possible and may be used for modeling the height distribution of ejecta in Europa's exosphere.Fil: Anders, Christian. Technische Universität Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. Technische Universität Kaiserslautern; Alemani

Controlled rippling of graphene via irradiation and applied strain modify its mechanical properties: a nanoindentation simulation study

Ripples, present in free standing graphene, have an important influence in the mechanical behavior of this two-dimensional material. In this work we show through nanoindentation simulations, how out-of-plane displacements can be modified by strain resulting in softening of the membrane under compression and stiffening under tension. Irradiation also induces changes in the mechanical properties of graphene. Interestingly, compressed samples, irradiated at low doses are stiffened by the irradiation while samples under tensile strain do not show significant changes in their mechanical properties. These simulations indicate that vacancies, produced by the energetic ions, cannot be the ones directly responsible for this behavior. However, changes in roughness induced by the momentum transferred from the energetic ions to the membrane, can explain these differences. These results provide an alternative explanation to recent experimental observations of stiffening of graphene under low dose irradiation, as well as paths to tailor the mechanical properties of this material via applied strain and irradiation.This work is supported by the Generalitat Valenciana through grant reference PROMETEO2012/011 and the Spanish government through grant FIS2010-21883. CJR and EMB thanks support from SeCTyP-UNCuyo grant M003, and ANPCyT grant PICT-2014-0696. CJR thanks CONICET and the 310 Group at FCEN-UNCuyo

Boosting materials science simulations by high performance computing

Technology development is often limited by knowledge of materials engineering and manufacturing processes. This scenario spans across scales and disciplines, from aerospace engineering to MicroElectroMechanical Systems (MEMS) and NanoElectroMechanical Systems (NEMS). The mechanical response of materials is dictated by atomic/nanometric scale processes that can be explored by molecular dynamics (MD) simulations. In this work we employ atomistic simulations to prove indentation as a prototypical deformation process showing the advantage of High Performance Computing (HPC) implementations for speeding up research. Selecting the right HPC hardware for executing simulations is a process that usually involves testing different hardware architectures and software configurations. Currently, there are several alternatives, using HPC cluster facilities shared between several researchers, as provided by Universities or Government Institutions, owning a small cluster, acquiring a local workstation with a high-end microprocessor, and using accelerators such as Graphics Processing Units (GPU), Field Programmable Gate Arrays (FPGA), or Intel Many Integrated Cores (MIC). Given this broad set of alternatives, we run several benchmarks using various University HPC clusters, a former TOP500 cluster in a foreign computing center, two high-end workstations and several accelerators. A number of different metrics are proposed to compare the performance and aid in the selection of the best hardware architecture according to the needs and budget of researchers. Amongst several results, we find that the Titan X Pascal GPU has a ∼3 x speedup against 64 AMD Opteron CPU cores.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

Nanoindentation tests of heavy-ion-irradiated Au foams - Molecular dynamics simulation

Irradiation by light ions may change the mechanical properties of nanofoams. Using molecular-dynamics simulation, we study the effect of irradiating a Au foam (porosity, 50%, and ligament diameter, 3 nm) with heavy ions: here, 10 keV Au ions up to a dose of 4 × 1016 m-2. We demonstrate that in consequence, the ligament morphology changes in the irradiated region, caused by local melting. The changes in mechanical properties are monitored by simulated nanoindentation tests. We find that the foam hardness is only around 1/3 of the hardness of a bulk Au crystal. Irradiation increases the hardness of the foam by around 10% in the central irradiated area. The plastic zone extends to only 1.5 ac, where ac denotes the contact radius; this value is unchanged under irradiation. The hardness increase after irradiation is attributed to two concurring effects. To begin with, irradiation induces melting and annealing of the ligaments, leading to their coarsening and alleviating surface stress, which in turn increases the dislocation nucleation threshold. In addition, irradiation introduces a stacking fault forest that acts as an obstacle to dislocation motion.Fil: Ruestes, Carlos Javier. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Anders, Christian. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; Alemani

Boosting materials science simulations by high performance computing

Technology development is often limited by knowledge of materials engineering and manufacturing processes. This scenario spans across scales and disciplines, from aerospace engineering to MicroElectroMechanical Systems (MEMS) and NanoElectroMechanical Systems (NEMS). The mechanical response of materials is dictated by atomic/nanometric scale processes that can be explored by molecular dynamics (MD) simulations. In this work we employ atomistic simulations to prove indentation as a prototypical deformation process showing the advantage of High Performance Computing (HPC) implementations for speeding up research. Selecting the right HPC hardware for executing simulations is a process that usually involves testing different hardware architectures and software configurations. Currently, there are several alternatives, using HPC cluster facilities shared between several researchers, as provided by Universities or Government Institutions, owning a small cluster, acquiring a local workstation with a high-end microprocessor, and using accelerators such as Graphics Processing Units (GPU), Field Programmable Gate Arrays (FPGA), or Intel Many Integrated Cores (MIC). Given this broad set of alternatives, we run several benchmarks using various University HPC clusters, a former TOP500 cluster in a foreign computing center, two high-end workstations and several accelerators. A number of different metrics are proposed to compare the performance and aid in the selection of the best hardware architecture according to the needs and budget of researchers. Amongst several results, we find that the Titan X Pascal GPU has a ∼3 x speedup against 64 AMD Opteron CPU cores.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

Stopping of porous projectiles in granular targets

Using granular mechanics, we determine the stopping force acting on spherical granular projectiles impinging on a flat granular bed. We find that the stopping force is proportional to the impact energy, as in Poncelet's law. For fixed velocity, it is proportional to the projectile cross-sectional area rather than to its volume. These dependences only hold in the early stages of stopping, before the projectile has been strongly fragmented. Analogies to the stopping of atomic clusters in compact matter are pointed out.Fil: Planes, María Belén. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Instituto para las Tecnologías de la Información y las Comunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

A Monte Carlo code for the collisional evolution of porous aggregates (CPA)

Context. The collisional evolution of submillimeter-sized porous dust aggregates is important in many astrophysical fields. Aims. We have developed a Monte Carlo code to study the processes of collision between mass-asymmetric, spherical, micron-sized porous silica aggregates that belong to a dust population. Methods. The Collision of Porous Aggregates (CPA) code simulates collision chains in a population of dust aggregates that have different sizes, masses, and porosities. We start from an initial distribution of granular aggregate sizes and assume some collision velocity distribution. In particular, for this study we used a random size distribution and a Maxwell-Boltzmann velocity distribution. A set of successive random collisions between pairs of aggregates form a single collision chain. The mass ratio, filling factor, and impact velocity influence the outcome of the collision between two aggregates. We averaged hundreds of thousands of independent collision chains to obtain the final, average distributions of aggregates. Results. We generated and studied four final distributions (F), for size (n), radius (R), porosity, and mass-porosity distributions, for a relatively low number of collisions. In general, there is a profuse generation of monomers and small clusters, with a distribution F (R) ∝ R-6 for small aggregates. Collisional growth of a few very large clusters is also observed. Collisions lead to a significant compaction of the dust population, as expected. Conclusions. The CPA code models the collisional evolution of a dust population and incorporates some novel features, such as the inclusion of mass-asymmetric aggregates (covering a wide range of aggregate radii), inter-granular friction, and the influence of porosity.Fil: Millán, Emmanuel Nicolás. Universidad Nacional de Cuyo. Instituto para las Tecnologías de la Informacion y las Comunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Planes, María Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Universidad de Mendoza. Facultad de Ingenieria; ArgentinaFil: Urbassek, Herbert M.. University Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad de Mendoza. Facultad de Ingenieria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentin

Effect of subsurface voids on the nanoindentation of Fe crystals

Subsurface voids may strongly affect the response of materials to nanoindentation. We explore these effects for a bcc single-crystalline Fe sample using molecular dynamics simulation. Deformation occurs mainly by nucleation and propagation of dislocations. As dislocations impinge into the voids, these suffer a reduction in volume, consistent with mass transfer mechanisms. Our results show that voids act as highly efficient absorbers of dislocations, effectively limiting the extension of the plastic zone. Surprisingly, mechanical properties are marginally affected by the presence of voids in the range of sizes and spatial distributions tested, except for voids a few nanometers below the surface. Deformation twinning is observed as a transient effect in some cases; however, for voids close enough to the indentation area, no twinning was found.Fil: Hofer, Juan Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ruestes, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad de Mendoza. Facultad de Ingeniería; Argentina. Universidad Mayor; ChileFil: Urbassek, Herbert M.. University Kaiserslautern; Alemani

Interplay of plasticity and phase transformation in shock wave propagation in nanocrystalline iron

Strong shock waves create not only plasticity in Fe, but also phase transform the material from its bcc phase to the high-pressure hcp phase. We perform molecular-dynamics simulations of large, 8-million atom nanocrystalline Fe samples to study the interplay between these two mechanisms. We compare results for a potential that describes dislocation generation realistically but excludes phase change with another which in addition faithfully features the bcc → hcp transformation. With increasing shock strength, we find a transition from a two-wave structure (elastic and plastic wave) to a three-wave structure (an additional phase-transformation wave), in agreement with experiment. Our results demonstrate that the phase transformation is preceded by dislocation generation at the grain boundaries (GBs). Plasticity is mostly given by the formation of dislocation loops, which cross the grains and leave behind screw dislocations. We find that the phase transition occurs for a particle velocity between 0.6 and 0.7 km s−1. The phase transition takes only about 10 ps, and the transition time decreases with increasing shock pressure.Fil: Gunkelman, Nina. University of Kaiserlautern; AlemaniaFil: Tramontina Videla, Diego Ramiro. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Urbassek, Herbert M.. University of Kaiserlautern; Alemani