Aconsegueixen desplaçar objectes nanomètrics mitjançant canvis de temperatura

A través d'un nanotub multicapa de carboni s'ha demostrat que l'origen directe del moviment d'un objecte mòbil es troba en l'heterogeneïtat tèrmica d'aquest tub. Apareix quan flueix un corrent elèctric per l'interior del tub conductor -amb el seu corresponent escalfament- i els dos electrodes dels seus extrems estan a temperatura ambient

First-principles simulations of lithium melting: Stability of the bcc phase close to melting

4 páginas, 2 figuras.We report large-scale first-principles simulations of melting of four different phases of Li at pressures ranging from 0 to 50 GPa. We find excellent agreement with existing experimental data at low pressures, and confirm that above 10 GPa the melting line develops a negative slope, in parallel to what occurs for Na at 30 GPa. Surprisingly, our results indicate that the melting temperature of the bcc phase is always higher than that of fcc Li, suggesting the intriguing possibility of the existence of a narrow field of bcc stability separating the fcc and liquid phases, as predicted by Alexander and McTague.The work of E. R. H. is supported by the Spanish Ministry of Science and Innovation through Project No. FIS2006-12117-C04-01; A. B. thanks GV-UPV/EHU for Project No. IT-366-07. The work of D. A. was conducted as part of an EURYI scheme grant as provided by EPSRC-GB.Peer reviewe

Behavior of Clayey Soil Existing in the Portoviejo Canton and Its Neutralization Characteristics

An investigation has been carried out on soil characterization and behavior in the canton of Portoviejo province of Manabí, in the Eloy Alfaro street sector of the Andrés de Vera Parish; where pits or open-air surveys were carried out and experimental laboratory tests of soils. It was demonstrated that soils have expansive properties of different characteristics, from medium to high degree of expansively according to the results of Laboratory of Soil Mechanics, where samples were tested analyzing the behavior according to the soil type. In this context, the results obtained by incorporating lime into the mixture are presented in 3 and 6%, thus obtaining a reduction by contraction to the volumetric change of the soil between 8 and 13%, allowing to improve its physical and mechanical properties; as well as its increase to bearing capacity by the California Bearing Ratio (CBR) test method from 9 to 16%, thus improving soil plasticity. With the results obtained, a description of the soil could be elaborated to determine the variations and changes according to the lime content, which was added to the soil, which allowed to reduce the deformations and damages caused to the superficial and structural pavement caused by the clays expansive, because in contact with the water the soil expands and at the same time when it drains the water contracts, proving that stabilizing the expansive clays with hydrated lime would improve the conditions of the pavement and its useful life

Path-integral molecular dynamics simulation of 3C-SiC

Molecular dynamics simulations of 3C-SiC have been performed as a function of pressure and temperature. These simulations treat both electrons and atomic nuclei by quantum mechanical methods. While the electronic structure of the solid is described by an efficient tight-binding Hamiltonian, the nuclei dynamics is treated by the path integral formulation of statistical mechanics. To assess the relevance of nuclear quantum effects, the results of quantum simulations are compared to others where either the Si nuclei, the C nuclei or both atomic nuclei are treated as classical particles. We find that the experimental thermal expansion of 3C-SiC is realistically reproduced by our simulations. The calculated bulk modulus of 3C-SiC and its pressure derivative at room temperature show also good agreement with the available experimental data. The effect of the electron-phonon interaction on the direct electronic gap of 3C-SiC has been calculated as a function of temperature and related to results obtained for bulk diamond and Si. Comparison to available experimental data shows satisfactory agreement, although we observe that the employed tight-binding model tends to overestimate the magnitude of the electron-phonon interaction. The effect of treating the atomic nuclei as classical particles on the direct gap of 3C-SiC has been assessed. We find that non-linear quantum effects related to the atomic masses are particularly relevant at temperatures below 250 K.Comment: 14 pages, 15 figure

Reversal of gulf stream circulation in a vertically vibrated triangular fluidized bed

Proceeding of: Fifteenth International Conference on Fluidization, Fluidization XV, Fluidization for Emerging Green Technologies, Montebello, Canada, May 22nd to 27th, 2016The present work experimentally assesses the effect of vibration on the dynamics of particles in a fluidized bed of triangular shape. The base of the bed is composed of two inclined walls, each one forming an angle of 45 with the horizontal. The bed has 0.206 m span and 0.01 m thickness. The bed vessel is made of antistatic PMMA in order to allow optical access with a high-speed camera. The bed is mounted on an electrodynamic shaker which produces vertical vibration. The bed material is ballotini particles with a mean diameter of 1.15 mm up to the top of the inclined walls. Air was injected through the inclined bed walls to fluidize the bed to explore whether vibration of the bed vessel together with gas injection can make the dynamics of this bed different to that found when no gas is injected. A high speed camera was used to record the motion of particles in the bed. The velocity of the particles in the bed was obtained via Particle Image Velocimetry (PIV). The results show that several circulation patterns are observed as a function of vibration amplitude and frequency when the fluidization velocity is just below and above the minimum fluidization velocity. Noticeably, for zero gas velocity, particles ascend close to the side walls and descend in the center of the bed. By injecting fluidization gas, the circulation pattern of the bed can be reversed (i.e. particles descending near the inclined walls and ascending in the center of the bed). Conditions for which this reversal of the gulf stream circulation of particles appears in the triangular bed are explored in this work and these include gas superficial velocities higher than the minimum fluidization velocity and sufficiently high values of the vibration strength.This work has been partially funded by the Universidad Carlos IIIde Madrid (Ayudas a la movilidad 2015) and by the Spanish Ministry of Economy and Competitiveness (project ENE2015/00188/001)

Hydrogen and muonium in diamond: A path-integral molecular dynamics simulation

Isolated hydrogen, deuterium, and muonium in diamond have been studied by path-integral molecular dynamics simulations in the canonical ensemble. Finite-temperature properties of these point defects were analyzed in the range from 100 to 800 K. Interatomic interactions were modeled by a tight-binding potential fitted to density-functional calculations. The most stable position for these hydrogenic impurities is found at the C-C bond center. Vibrational frequencies have been obtained from a linear-response approach, based on correlations of atom displacements at finite temperatures. The results show a large anharmonic effect in impurity vibrations at the bond center site, which hardens the vibrational modes with respect to a harmonic approximation. Zero-point motion causes an appreciable shift of the defect level in the electronic gap, as a consequence of electron-phonon interaction. This defect level goes down by 70 meV when replacing hydrogen by muonium.Comment: 11 pages, 8 figure

Segregation of equal-sized particles of different densities in a vertically vibrated fluidized bed

Proceeding of: Fifteenth International Conference on Fluidization, Fluidization XV, Fluidization for Emerging Green Technologies, Montebello, Canada, May 22nd to 27th, 2016The present work experimentally studies the influence of vibration and gas velocity on the density-induced segregation of particles in a pseudo-2D vibrated fluidized bed. One half of the particles of the bed are ballotini spheres of density 2500 kg/m(3) and the other half are heavier ceramic particles of density 4100 kg/m(3) or 6000 kg/m(3). Digital Image Analysis is used to characterize the rate and extent of particle mixing with time for different gas velocities, vibration amplitudes and frequencies. The results of the experiments indicate that the vibration strength and the gas velocity have an important effect on both the evolution and the final extent of density-induced particle segregation. It was observed that by introducing vertical vibration to a bed that is fluidized close to minimum fluidization conditions the rate of segregation and the final segregation index of a mixture of light and dense particles is enhanced. However, for vibration strengths greater than a critical value around 3-4, the degree of segregation decreases due to a more vigorous three dimensional mixing of particles in the bed.This work has been partially funded by the Universidad Carlos III de Madrid (Ayudas a la movilidad 2015) and by the Spanish Ministry of Economy and Competiveness (project ENE2015/00188/001)

Theoretical evidence for the kick-out mechanism for B diffusion in SiC

In this letter, we analyze by means of first-principles electronic structure calculations the diffusion of B impurities in 3C-SiC. We find, through molecular dynamics, that substitutional B at a Si lattice site is readily displaced by a nearby Si interstitial by the process known as a kick-out mechanism, in agreement with recent experimental results. This is in contrast to the situation in Si, where B has recently been shown to diffuse via an interstitialcy mechanism

First-principles study of n-type dopants and their clustering in SiC

We report the results of an ab initio study of N and P dopants in SiC. We find that while N substitutes most favorably at a C lattice site, P does so preferably at a Si site, except in n-doping and Si-rich 3C-SiC. Furthermore, we consider a series of dopant complexes that could form in high-dose implantation, in order to investigate the dopant activation behavior in this limit. We find that all N complexes considered lead to passivation through the formation of a deep level. For P, the most stable aggregate is still an active dopant, while passivation is only observed for complexes with a higher formation energy. We discuss how these results could help in the understanding of the observed experimental high-dose doping and codoping behavior of these species