Role of Layering Oscillations at Liquid Metal Surfaces in Bulk Recrystallization and Surface Melting

The contrasting melting behavior of different surface orientations in metals can be explained in terms of a repulsive or attractive effective interaction between the solid-liquid and the liquid-vapor interface. We show how a crucial part of this interaction originates from the layering effects near the liquid metal surface. Its sign depends on the relative tuning of layering oscillations to the crystal interplanar spacing, thus explaining the orientational dependence. Molecular dynamics recrystallization simulations of Au surfaces provide direct and quantitative evidence of this phenomenon.Comment: 10 pages (RevTeX) plus 3 figures (PostScript

Contact of Single Asperities with Varying Adhesion: Comparing Continuum Mechanics to Atomistic Simulations

Atomistic simulations are used to test the equations of continuum contact mechanics in nanometer scale contacts. Nominally spherical tips, made by bending crystals or cutting crystalline or amorphous solids, are pressed into a flat, elastic substrate. The normal displacement, contact radius, stress distribution, friction and lateral stiffness are examined as a function of load and adhesion. The atomic scale roughness present on any tip made of discrete atoms is shown to have profound effects on the results. Contact areas, local stresses, and the work of adhesion change by factors of two to four, and the friction and lateral stiffness vary by orders of magnitude. The microscopic factors responsible for these changes are discussed. The results are also used to test methods for analyzing experimental data with continuum theory to determine information, such as contact area, that can not be measured directly in nanometer scale contacts. Even when the data appear to be fit by continuum theory, extracted quantities can differ substantially from their true values

Electronic Properties of Ultra-Thin Aluminum Nanowires

We have carried out first principles electronic structure and total energy calculations for a series of ultrathin aluminum nanowires, based on structures obtained by relaxing the model wires of Gulseren et al. The number of conducting channels is followed as the wires radius is increased. The results suggest that pentagonal wires should be detectable, as the only ones who can yield a channel number between 8 and 10.Comment: 9 pages + 3 figures, to appear on Surface Scienc

The Puzzling Stability of Monatomic Gold Wires

We have examined theoretically the spontaneous thinning process of tip-suspended nanowires, and subsequently studied the structure and stability of the monatomic gold wires recently observed by Transmission Electron Microscopy (TEM). The methods used include thermodynamics, classical many-body force simulations, Local Density (LDA) and Generalized Gradient (GGA) electronic structure calculations as well as ab-initio simulations including the two tips. The wire thinning is well explained in terms of a thermodynamic tip suction driving migration of surface atoms from the wire to the tips. For the same reason the monatomic wire becomes progressively stretched. Surprisingly, however, all calculations so far indicate that the stretched monatomic gold wire should be unstable against breaking, contrary to the apparent experimental stability. The possible reasons for the observed stability are discussed.Comment: 4 figure

Molecular dynamics study of melting of a bcc metal-vanadium II : thermodynamic melting

We present molecular dynamics simulations of the thermodynamic melting transition of a bcc metal, vanadium using the Finnis-Sinclair potential. We studied the structural, transport and energetic properties of slabs made of 27 atomic layers with a free surface. We investigated premelting phenomena at the low-index surfaces of vanadium; V(111), V(001), and V(011), finding that as the temperature increases, the V(111) surface disorders first, then the V(100) surface, while the V(110) surface remains stable up to the melting temperature. Also, as the temperature increases, the disorder spreads from the surface layer into the bulk, establishing a thin quasiliquid film in the surface region. We conclude that the hierarchy of premelting phenomena is inversely proportional to the surface atomic density, being most pronounced for the V(111) surface which has the lowest surface density

Premelting of Thin Wires

Recent work has raised considerable interest on the nature of thin metallic wires. We have investigated the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential. We find that---in analogy with cluster melting---the melting temperature $T_m (R)$ of a wire with radius $R$ is lower than that of a bulk solid, $T_m^b$, by $T_m (R) = T_m^b -c/R$. Surface melting effects, with formation of a thin skin of highly diffusive atoms at the wire surface, is observed. The diffusivity is lower where the wire surface has a flat, local (111) orientation, and higher at (110) and (100) rounded areas. The possible relevance to recent results on non-rupturing thin necks between an STM tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref. 131/94/CM/S

Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium

We report x-ray reflectivity (XR) and small angle off-specular diffuse scattering (DS) measurements from the surface of liquid Indium close to its melting point of $156^\circ$C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron density profile displays exponentially decaying surface induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron density profiles of liquid In, which is a nearly free electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of $3.5\pm 0.6$ \AA for In and $5.5\pm 0.4$ \AA for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.

Otimização do processo de pelotização visando o incremento da resistência mecânica das pelotas

The excessive fine particles in the calcined ore is one of the main aspects that limit the increase of production, result in energy inefficiency and instabilities in the processes. The pelletizing plant is responsible for treating, in closed circuit, the dust generated in electric furnaces and rotary kilns, guaranteeing the metallic recovery, because the kiln dust contains on average 2.4% of nickel. The survival of the pellets to the rotary kiln is essential for stability, productivity and metal recovery. This work had as main objective the determination of the optimum point of operation of the pelletizing disks, aiming to increase the mechanical resistance (kgf) of the pellets using the statistical design and analysis of experiments. Through a factorial experiment with two factors: with a disc speed levels in% RPM (factor A), b feed rate levels in t/h (factor B), n replicates and each replica containing all combinations of factors A (velocity of the disk) and B (feed rate), the ANOVA was performed for factor A, factor B and interaction between them. After the ANOVA, it was possible to infer, through the Tukey multiple comparison methods, the best point of operation. The best operating condition was the replicated to all four pelletizing discs. After three months of operation in the optimized condition, it was possible to verify the reduction of around 3% in the recirculation of kiln dust. The reduction of the circulating kiln dust load due to the increase in pellet resistance allowed the production to increase by 114 t nickel/year, net adds of USD 365,545/year, a reduction of 3.200 m3 of water/year, 11.7 thousand GJ/year and a reduction of 1,5 thousand tons of CO2eq/year.A presença excessiva de partículas finas no minério calcinado é um dos principais aspectos que limitam o aumento da produção, gerando ineficiência energética e instabilidades nos processos. A pelotização é responsável por tratar, em circuito fechado, todo o pó gerado nos fornos elétricos e calcinadores, garantindo a recuperação metálica, uma vez que o pó contém em média 2,4% de níquel. Assim, a sobrevivência das pelotas ao processo de calcinação é fundamental para estabilidade, produtividade e recuperação metálica. Este trabalho teve como objetivo a determinação do ponto ótimo de operação dos discos pelotizadores, visando o aumento da resistência mecânica (kgf) das pelotas. Para isso foram utilizadas as ferramentas estatísticas de planejamento e análise de experimentos. Através de um experimento fatorial com a níveis da velocidade do disco em % RPM (fator A), b níveis da taxa de alimentação em t/h (fator B), n réplicas, sendo que, cada réplica contém todas as combinações dos fatores A e B. Foi realizada a análise da variância (ANOVA) para o fator A, o fator B e interação entre eles. Após a ANOVA, foi possível inferir, através dos métodos de comparações múltiplas de Tukey o melhor ponto de operação. Em seguida essa condição de operação foi replicada para todos os discos pelotizadores. Após três meses de operação na condição otimizada, foi comprovada a redução de 3% na recirculação de pó. A redução da carga circulante de pó em decorrência do aumento da resistência das pelotas possibilitou o incremento da produção em 114 t de níquel/ano, ganho líquido de 1.3 milhões de Reais/ano, redução de 3,2 mil m3 de água/ano, 11,7 mil GJ/ano e redução de 1,5 mil toneladas de CO2eq/an

Structure and stability of finite gold nanowires

Finite gold nanowires containing less than 1000 atoms are studied using the molecular dynamics simulation method and embedded atom potential. Nanowires with the face-centered cubic structure and the (111) oriented cross-section are prepared at T=0 K. After annealing and quenching the structure and vibrational properties of nanowires are studied at room temperature. Several of these nanowires form multi-walled structures of lasting stability. They consist of concentrical cylindrical sheets and resemble multi-walled carbon nanotubes. Vibrations are investigated by diagonalization of the dynamical matrix. It was found that several percents of vibrational modes are unstable because of uncompleted restructuring of initial fcc nanowires.Comment: 4 figures in gif forma

DEPRESSÃO EM PACIENTES IDOSOS COM DOENÇA DE ALZHEIMER

Esta pesquisa trata do conhecimento e análise dos desafios vivenciados no processo de envelhecimento e nos impactos sociais na vida do idoso. A relação entre o desenvolvimento da doença de Alzheimer e a depressão, envelhecimento e declínio da memória foram investigados. O desenvolvimento desta pesquisa se deu por meio da análise de banco de dados do estudo Alzheimer's Disease Neuroimaging Initiative (ADNI), além de exame de cenários abrangentes de estudos sobre a doença de Alzheimer no Brasil e no mundo. A metodologia utilizada baseou-se na definição da base de dados ideal a ser analisada, assim como a sua complementação, tratamento e estratificação. A análise estatística comparativa foi elaborada a partir dos dados provenientes de dois subgrupos (com e sem depressão) de participantes do ADNI que iniciaram este estudo com comprometimento cognitivo leve e terminaram com doença de Alzheimer. Foram avaliadas as informações demográficas, as taxas de conversão de participantes com comprometimento cognitivo leve para doença de Alzheimer, biomarcadores de imagem, biomarcadores químicos e testes de avaliação cognitiva, além de avaliações globais, comportamentais e funcionais. Os resultados obtidos, após as análises, confirmaram que os participantes acometidos de depressão são mais impactados no desenvolvimento da doença de Alzheimer que os participantes sem depressão