Molecular dynamics of nanodroplet impact: The effect of the projectile’s molecular mass on sputtering

The impact of electrosprayed nanodroplets on ceramics at several km/s alters the atomic order of the target, causing sputtering, surface amorphization and cratering. The molecular mass of the projectile is known to have a strong effect on the impact phenomenology, and this article aims to rationalize this dependency using molecular dynamics. To achieve this goal, the article models the impact of four projectiles with molecular masses between 45 and 391 amu, and identical diameters and kinetic energies, 10 nm and 63 keV, striking a silicon target. In agreement with experiments, the simulations show that the number of sputtered atoms strongly increases with molecular mass. This is due to the increasing intensity of collision cascades with molecular mass: when the fixed kinetic energy of the projectile is distributed among fewer, more massive molecules, their collisions with the target produce knock-on atoms with higher energies, which in turn generate more energetic and larger numbers of secondary and tertiary knock-on atoms. The more energetic collision cascades intensify both knock-on sputtering and, upon thermalization, thermal sputtering. Besides enhancing sputtering, heavier molecules also increase the fraction of the projectile’s energy that is transferred to the target, as well as the fraction of this energy that is dissipated

Coherent phonon transport in short-period two-dimensional superlattices of graphene and boron nitride

Promoting coherent transport of phonons at material interfaces is a promising strategy for controlling thermal transport in nanostructures and an alternative to traditional methods based on structural defects. Coherent transport is particularly relevant in short-period heterostructures with smooth interfaces and long-wavelength heat-carrying phonons, such as two-dimensional superlattices of graphene and boron nitride. In this work, we predict phonon properties and thermal conductivities in these superlattices using a normal mode decomposition approach. We study the variation of the frequency dependence of these properties with the periodicity and interface configuration (zigzag and armchair) for superlattices with period lengths within the coherent regime. Our results showed that the thermal conductivity decreases significantly from the first period length (0.44 nm) to the second period length (0.87 nm), 13% across the interfaces and 16% along the interfaces. For greater periods, the conductivity across the interfaces continues decreasing at a smaller rate of 11 W/mK per period length increase (0.43 nm), driven by changes in the phonon group velocities (coherent effects). In contrast, the conductivity along the interfaces slightly recovers at a rate of 2 W/mK per period, driven by changes in the phonon relaxation times (diffusive effects). By changing the interface configuration from armchair to zigzag, the conductivities for all period lengths increase by approximately 7% across the interfaces and 19% along the interfaces

Interactions and thermoelectric effects in a parallel-coupled double quantum dot

We investigate the nonequilibrium transport properties of a double quantum dot system connected in parallel to two leads, including intradot electron-electron interaction. In the absence of interactions the system supports a bound state in the continuum. This state is revealed as a Fano antiresonance in the transmission when the energy levels of the dots are detuned. Using the Keldysh nonequilibrium Green's function formalism, we find that the occurrence of the Fano antiresonance survives in the presence of Coulomb repulsion. We give precise predictions for the experimental detection of bound states in the continuum. First, we calculate the differential conductance as a function of the applied voltage and the dot level detuning and find that crossing points in the diamond structure are revealed as minima due to the transmission antiresonances. Second, we determine the thermoelectric current in response to an applied temperature bias. In the linear regime, quantum interference gives rise to sharp peaks in the thermoelectric conductance. Remarkably, we find interaction induced strong current nonlinearities for large thermal gradients that may lead to several nontrivial zeros in the thermocurrent. The latter property is especially attractive for thermoelectric applications.Comment: 9 pages, 8 figure

Enhancing thermoelectric properties of graphene quantum rings

We study the thermoelectric properties of rectangular graphene rings connected symmetrically or asymmetrically to the leads. A side-gate voltage applied across the ring allows for the precise control of the electric current flowing through the system. The transmission coefficient of the rings manifests Breit-Wigner line-shapes and/or Fano line-shapes, depending on the connection configuration, the width of nanoribbons forming the ring and the side-gate voltage. We find that the thermopower and the figure of merit are greatly enhanced when the chemical potential is tuned close to resonances. Such enhancement is even more pronounced in the vicinity of Fano like anti-resonances which can be induced by a side-gate voltage independently of the geometry. This opens a possibility to use the proposed device as a tunable thermoelectric generator.Comment: 6 pages, 5 figures, accepted for publication in Physical Review

Lattice thermal conductivity of graphene nanostructures

Non-equilibrium molecular dynamics is used to investigate the heat current due to the atomic lattice vibrations in graphene nanoribbons and nanorings under a thermal gradient. We consider a wide range of temperature, nanoribbon widths up to 6nm and the effect of moderate edge disorder. We find that narrow graphene nanorings can efficiently suppress the lattice thermal conductivity at low temperatures (~100K), as compared to nanoribbons of the same width. Remarkably, rough edges do not appear to have a large impact on lattice energy transport through graphene nanorings while nanoribbons seem more affected by imperfections. Furthermore, we demonstrate that the effects of hydrogen-saturated edges can be neglected in these graphene nanostructures

Conservation Laws in Smooth Particle Hydrodynamics: the DEVA Code

We describe DEVA, a multistep AP3M-like-SPH code particularly designed to study galaxy formation and evolution in connection with the global cosmological model. This code uses a formulation of SPH equations which ensures both energy and entropy conservation by including the so-called \bn h terms. Particular attention has also been paid to angular momentum conservation and to the accuracy of our code. We find that, in order to avoid unphysical solutions, our code requires that cooling processes must be implemented in a non-multistep way. We detail various cosmological simulations which have been performed to test our code and also to study the influence of the \bn h terms. Our results indicate that such correction terms have a non-negligible effect on some cosmological simulations, especially on high density regions associated either to shock fronts or central cores of collapsed objects. Moreover, they suggest that codes paying a particular attention to the implementation of conservation laws of physics at the scales of interest, can attain good accuracy levels in conservation laws with limited computational resources.Comment: 36 pages, 10 figures. Accepted for publication in The Astrophysical Journa

Utilización de arenas procedentes de residuos de construcción y demolición, rcd, en la fabricación de morteros de albañilería.

En esta investigación se estudió el comportamiento y las propiedades más relevantes de los morteros de albañilería, al sustituir parte de la arena convencional por arena reciclada en diferentes porcentajes. Para ello se caracterizaron las arenas recicladas procedentes de las centrales de reciclaje y se realizaron ensayos con el fin de comparar las propiedades de los morteros tradicionales con los morteros elaborados con arena reciclada. La dosificación del mortero utilizada ha sido 1:3:0,5, y los porcentajes de sustitución de arena normalizada por arena reciclada fueron de: 10%, 15%, 25%, 35% y 45% , para los tres tipos de arenas recicladas empleadas en este estudio, dos arenas procedentes de la línea de reciclaje de hormigón y una de la línea de reciclaje cerámica. Los resultados indican un aumento en la demanda de agua para obtener morteros trabajables, siendo necesario el empleo de aditivos a partir del 10% de sustitución. Las resistencias mecánicas obtenidas en los morteros reciclados son superiores en el caso de la flexión y ligeramente inferiores en compresión. Tanto en el ensayo de adherencia como en el de retracción, los morteros reciclados presentan un peor comportamiento que los morteros tradicionales, pero sin evidenciar pérdidas significativas en dichos valores, por lo que los morteros de cemento para albañilería con dosificación 1:3:0,5 pueden incorporar hasta un 45% de árido reciclado sin que sus principales características se vean afectada

Caracterización e influencia del arido reciclado fino en las propiedades de los morteros de albañilería

This research aims to study mechanical behaviour and relevant properties of masonry mortars fabricated using fine recycled aggregate in different mixture proportions. Fine recycled aggregates samples originated from the ceramic and concrete recycling process and coming from two recycling plants of Madrid region have been used. Tests were performed using 1:3:0.5 volumetric cement-to-aggregate-to-water ratio. Standardized sand with fine recycled aggregate replacement percentages were: 10%, 15%, 25%, 35% and 45%. A continuous size distribution curve can be observed and the main crystalline phases determined have been quartz, calcite and gypsum. Compressive strength, shrinkage and bond strength tests revealed poorer performance of recycled mortars compared to the conventional mortars; however, specific values are within the limits established by the manufacturers and standards. This study shows that cement-based mortars prepared with volumetric ratio 1:3:0.5 may contain up to 45% of fine recycled aggregates, without their properties being affected and without presenting significant losses.Esta investigación estudia el comportamiento mecánico y las propiedades más relevantes de los morteros de albañilería fabricados usando arenas recicladas en diferentes proporciones. Muestras pertenecientes a la línea de reciclaje cerámica y de hormigón proceden de dos centrales de reciclaje de la Comunidad de Madrid. Los ensayos se realizaron con una dosificación 1:3:0,5. Los porcentajes de arena reciclada fueron: 10%, 15%, 25%, 35% y 45%. Se observa una línea granulométrica continua y las principales fases cristalinas encontradas son cuarzo, calcita y yeso. Los ensayos de resistencia a compresión, retracción y adherencia muestran un peor comportamiento en los morteros reciclados frente a los morteros elaborados con arena normalizada, aunque dentro de los límites establecidos por normativas y fabricantes. Se deduce que, los morteros de albañilería base cemento, pueden incorporar un 45% de arena reciclada con una dosificación 1:3:0,5 sin que las propiedades ensayadas se vean afectadas con pérdidas significativas de prestaciones