Da ordem à desordem : uma visao da ciência dos materiais computacional

Orientador: Alex AntonelliTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Os conceitos de ordem e desordem em materiais foram explorados sob a ótica de simulações computacionais. O objetivo foi mostrar que, a partir de simulações computacionais, é possível modelar, caracterizar e prever as propriedades e processos relacionados à ordem & desordem em materiais nas suas várias dimensões. A modelagem computacional se deu através de duas metodologias: cálculos de primeiros princípios e potenciais empíricos utilizando o método de Monte Carlo. Tendo como paradigma de ordem perfeita os semicondutores na estrutura do diamante, abordamos tanto os elementos de ordem na desordem, quanto os de desordem na ordem, além de introduzirmos o conceito de supra-ordem. Em relação aos elementos de ordem na desordem, abordamos quantitativamente o problema da transição vítrea no Si e das propriedades do Si amorfo. A respeito da supra-ordem, as propriedades termodinâmicas, estruturais e energéticas dos Clatratos tipos I e II do Si, Ge e C foram determinadas. Pela primeira vez determinou-se os pontos de fusão desses sistemas para o Si: Si34 (1522 K) e Si46 (1482 K). Os estudos da energia livre a partir do método Reversible Scaling permitiram uma determinação acurada do diagrama de fases do Si e demonstrando a importância dos efeitos anarmônicos. Sobre os elementos de desordem na ordem, estudamos defeitos pontuais, lineares e planares. Em relação aos pontuais, estudamos as vacâncias em semicondutores nas estruturas do diamante, clatratos e amorfos. Em particular para os amorfos, investigamos os efeitos da relaxação estrutural sobre as propriedades eletrônicas através de cálculos ab initio. Um comportamento bastante rico foi observado, desde aniquilação da vacância à criação de defeitos "estáveis". Para os sítios estudados, os estados profundos do gap desaparecem com a relaxação atômica. A respeito dos defeitos lineares, abordamos o problema das diferenças de energias livres entre duas estruturas candidatas a caroço na discordância parcial de 900: Single Period (SP) e Double Period (DP). Os resultados indicam que a diferença média da energia livre entre as reconstruções, aumenta com o aumento da temperatura, tanto para o Si quanto Ge. Tomando a estrutura DP ainda mais dominante em relação a SP em altas temperaturas. Finalmente, estudamos os defeitos planares em Carbono na estrutura do diamante simulando o processo de cisalhamento. Sugerimos um novo modelo para os defeitos conhecidos como plaquetas e demonstramos que esse modelo satisfaz às propriedades experimentais conhecidasAbstract: Concepts of order and disorder in materials are explored from the point of view of computer simulations. Our main aim is to illustrate that is possible to model, characterize and predict the properties and processes related to order-disorder in materials in several degrees. Computational modeling has been performed by using ftrst principles calculations and Monte Carlo method with empirical potentials. Semiconductors in the diamond structure have been defined as the ideal ordered system. According to the present view, we have studied the symptoms of order in disordered systems and the symptoms of disorder in ordered systems, as well; we have introduced the concept of supra-order. About order in disorder, we have investigated the glass transition in Si and the properties of amorphous Si (a-Si) have been determinated. For the supra-order degree, the thermodynamics and structural properties of group-IV Clathrates have been studied. The melting point was estimated to be 1482 K for Si46 and 1522 K for Si34. The thermodynamic properties of Si phases as functions of temperature have been studied by using the Reversible Scaling Method in the Monte Carlo approach. We present a quantitative phase diagram of Silicon and we show that the anharmonic effects play an important role. On disorder in the order end, we have investigated point, linear and planar defects in materials. We have studied vacancies in diamond, clathrates and amorphous structures of semiconductors. In particular, for vacancies a-Si, a very rich behavior has been observed from the complete rearrangement of the atoms in order to preserve the tetrahedral structure to the creation of stable vacancies. We have also investigated the effects of structural relaxation of the electronic properties. From the electronic structure point of view, deep gap levels disappear after the structural relaxations in vacancies in a-Si. We have studied the differences of free energy between the single period and the double period core reconstructions of the 90° partial dislocation in sihcon and germanium as functions of temperature. Our results indicate that the average differences of free energy increase with temperature for both, Si and Ge, making the double period reconstruction even more dominant in high temperatures. Finally, we have simulated the shear process in Carbon diamond. We propose a new model for platelets in diamond that is in agreement with all experimental data availableDoutoradoFísicaDoutor em Ciência

Entropia vibracional em ligas metálicas : determinação de parâmetros termodinâmicos em ligas metálicas via ligação adiabática e dinâmica molecular

Orientador: Alex AntonelliDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: As propriedades termodinâmicas das ligas foram calculadas usando o método de Ligação Adiabática no formalismo da Dinâmica Molecular. Nós estudamos a aplicação do método da Ligação Adiabática na investigação do fenômeno ordem-desordem e defeitos pontuais na liga Ni3Al. Para as simulações de Dinâmica molecular usamos a dinâmica da Cadeia Massiva de Nosé-Hoover e o método de Andersen e para descrever as interações entre os átomos da liga usamos o potencial do tipo tight-binding de Cleri-Rosato. Quanto ao fenômeno ordem-desordem foram calculadas as diferenças ordem-desordem da energia livre e entropia vibracional da liga Ni3Al. Nós encontramos que a diferença ordem-desordem da entropia vibracional aumenta com a temperatura variando de 0,14 kb/átomo em 300 K até 0,21 kb/átomo em 1200 K. Estes resultados estão em concordância com os resultados experimentais. Os cálculos sugerem que o principal fator no aparecimenteo desta diferença ordem-desordem da entropia vibracional é a diferença entre os volumes da liga nas fases ordenada e desordenada. Calculamos as energias livres e entropias de fornmação das vacâncias e as energias de formação de vacâncias e anti-sítios para a liga Ni3Al na fase ordenada, esses resultados estão em boa concordância com resultados conhecidos. Em particular para a entropia vibracional de formação de vacâncias de Ni (2,7 kb) e Al (4,0 kb) na liga, nossos resultados por incluirem efeitos de anarmonicidade mostram-se mais realísticos que os resultados teóricos obtidos pelo método Quase-harmônico, além de concordarem com recentes resultados experimentais obtidos por Badura-Gergen e Schaefer, PRB 56,3032 (1997). A partir do cálculo das energia e entropia de formação fizemos um estudos da concentrações dos defeitos pontuais na liga Ni3Al em sua fase ordenada em 1000 K em função da concentração de Ni e com a temperatura para três composições diferentes da liga (Ni76Al74Ni75Al25 e Ni74Al26)Abstract: The thermodynamics properties of alloys are calculated employing the method of Adiabatic Switching in the Molecular Dynamics (MD) formalism. We study the application of the Adiabatic Switching to investigate order-disorder phenomena and point defects in Ni3Al. The MD simulations were performed using the Massive Nosé-Hoover Chain (MNHC) and Andersen dynamics and we have employed a tight-binding potential of Cleri and Rosato to describe interactions in the Ni-Al system. The free energy and entropy differences in Ni3Al between its equilibrium ordered structure and a disordered solid solution were calculated. We find that the vibration entropy difference increases with temperature from 0.14 kB/atom at 300 K to 0.21 kB/atom at 1200 K. These results are in agreement with experimental values. Our calculations suggested that the major cause of the entropy difference in this system is the volume difference between the ordered and disordered phases. The vacancy formation free energy and vibration entropies, vacancy and antisite defect formation energies and their corresponding relaxation volumes were evaluated in Ni3Al in the ordered phase, these values being in good agreements with known values. In particular, our results of Ni (2.7 kB) and Al (4.0 kB) vacancies formation entropies that include anharmonic effects are more realistic those by Quasi-harmonic method and agree with recent experimental data estimated by Badura-Gergen e Schaefer, PRB 56,3032 (1997). The concentration of point defects at 1000K as a function of Ni content and the effect of temperature on them were studied for three compositions (Ni76Al74,Ni75Al25 and Ni74Al26)MestradoFísicaMestre em Físic

Mechanical and adsorption properties of greenhouse gases filled carbon nanotubes

We investigate the mechanical and adsorption properties of single-walled carbon nanotubes (SWCNTs) filled with greenhouse gases through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations using a recently developed parameterization for the cross-terms of the Lenard-Jones (LJ) potential. Carbon nanotubes interact strongly with CO$_2$ compared to CH$_4$, resulting in a CO$_2$-rich composition inside the nanotubes, with the proportion of CO$_2$ decreasing as the diameter of the nanotubes increases. Contrarily, the smallest nanotubes showed a more even balance between CO$_2$ and CH$_4$ due to gas solidification. The gas does not affect the mechanical response of the nanotubes under tension, but under compression, it presents a complex relationship with the loading direction, nanotube's diameters, chirality, and to a minor extent, the gas composition. Filled zigzag nanotubes showed to be more stable in the presence of fillers, giving the best mechanical performance compared to the filled armchairs. The study confirms carbon nanotubes as effective means of separating CO$_2$ from CH$_4$, presenting good mechanical stability

Revisiting greenhouse gases adsorption in carbon nanostructures: advances through a combined first-principles and molecular simulation approach

Carbon nanotubes and graphene are promising nanomaterials to improve the performance of current gas separation membrane technologies. From the molecular modeling perspective, an accurate description of the interfacial interactions is mandatory to understand the gas selectivity in these materials. Most of the molecular dynamics simulations studies considered available force fields with the standard Lorentz-Berthelot (LB) mixing rules to describe the interaction among carbon dioxide (CO$_2$), methane (CH$_4$) and carbon structures. We performed a systematic study in which we showed the LB underestimates the fluid/solid interaction energies compared to the density functional theory (DFT) calculation results. To improve the classical description, we propose a new parametrization for the cross-terms of the Lenard-Jones (LJ) potential by fitting DFT forces and energies. The obtained model enhanced fluid/carbon interface description showed excellent transferability between single-walled carbon nanotubes (SWCNTs) and graphene. To investigate the effect of the new parametrization on the gas structuring within the SWCNTs with varying diameters, we performed Grand Canonical Monte Carlo (GCMC) simulations. We observed considerable differences in the CO$_2$ and CH$_4$ density within SWCNTs compared to those obtained with the standard approach. Our study highlights the importance of going beyond the traditional Lorentz-Berthelot mixing rules in the studies involving solid/fluid interfaces of confined systems

Fresh Molecular Look at Calcite-Brine Nanoconfined Interfaces

Calcite-fluid interface plays a central role in geochemical, synthetic, and biological crystal growth. The ionic nature of the calcite surface can modify the fluid-solid interaction and the fluid properties under spatial confinement and can also influence the adsorption of chemical species. We investigate the structure of the solvent and ions (Na, Cl, and Ca) at the calcite-aqueous solution interface under confinement and how such environment modifies the properties of water. To properly investigate the system, molecular dynamics simulations were employed to analyze the hydrogen bond network and to calculate NMR relaxation times. Here, we provide a new insight with additional atomistically detailed analysis by relating the topology of the hydrogen bond network with the dynamical properties in nanoconfinement interfaces. We have shown that the strong geometrical constraints and the presence of ions do influence the hydrogen bond network, resulting in more extended geodesic paths. Hydrogen bond branches connect low to high dynamics molecules across the pore and hence may explain the gluelike mechanical properties observed in the confinement environment. Moreover, we showed that the surface water observed at the calcite interface is characterized by slow transversal spin relaxation time (T2) and highly coordinated water molecules. The physical and electrostatic barrier emerged from the epitaxial ordering of water results in a particular ionic distribution, which can prevent the direct adsorption of a variety of chemical species. The implications of our results delineate important contributions to the current understanding of crystallization and biomineralization processes.Fil: Kirch, Alexsandro. Universidade de Sao Paulo; BrasilFil: Mutisya, Sylvia Mueni. Universidade Federal Do Abc; BrasilFil: Sanchez, Veronica Muriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Centro de Simulación Computacional para Aplicaciones Tecnológicas; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: de Almeida, James Moraes. Universidade de Sao Paulo; BrasilFil: Miranda, Caetano Rodrigues. Universidade de Sao Paulo; Brasi

H2O and CO2 Confined in Cement based Materials: An Ab Initio Molecular Dynamics study with van der Waals Interactions.

Although the cement has been widely used for a long time, very little is known regarding the atomistic mechanism behind its functionality. Particularly, the dynamics of molecular systems at confined nanoporous and water hydration is largely unknown. Here, we study the dynamical and structural properties of H2O and CO2 confined between Tobermorite 9A(T9) surfaces with Car-Parrinello molecular dynamics with and without van der Waals (vdW) interactions, at room temperature. For H2O confined, we have observed a broadening in the intra and intermolecular bond angle distribution. A shift from an ice-like to a liquid-like infrared spectrum with the inclusion of vdW interactions was observed. The bond distance for the confined CO2 was increased, followed with the appearance of shorter (larger) intramolecular (intermolecular) angles. These structural modifications result in variations on the CO2 symmetric stretching Raman active vibration modes. The diffusion coefficient obtained for both confined H2O and CO2 were found to be lower than their bulk counterparts. Interestingly, during the water dynamics, a proton exchange between H2O and the T9 surface was observed. However, for confined CO2, no chemical reactions or bond breaking were observed

Microscopia eletrônica de varredura da interação entre Cryptococcus magnus e Colletotrichum gloeosporioides em frutos de mamão

ABSTRACTThe objective of this work was to investigate possible modes of action of the yeast Cryptococcus magnus in controlling anthracnose (Colletotrichum gloeosporioides) on post harvested papaya fruits. Scanning electron microscopy was used to analyze the effect of the yeast on inoculations done after harvest. Results showed that C. magnus is able to colonize wound surfaces much faster than the pathogen, outcompeting the later for space and probably for nutrients. In addition, C. magnus produces a flocculent matrix, which affects hyphae integrity. The competition for space and the production of substances that affect hyphae integrity are among the most important modes of action of this yeast. ______________________________________________________________________________ RESUMOO objetivo deste trabalho foi investigar prováveis modos de ação da levedura Cryptococcus magnus, que resultam no controle da antracnose (Colletotrichum gloeosporioides) em frutos de mamoeiro na póscolheita. A microscopia eletrônica de varredura foi utilizada para avaliar o efeito da levedura sobre inoculações realizadas após a colheita. Os resultados mostraram que C. magnus é capaz de colonizar a superfície de ferimentos nos frutos e vencer a competição por espaço e, provavelmente, por nutrientes. Além disso, C. magnus produz uma matriz de textura característica que afeta a integridade da hifa do patógeno. A competição por espaço e a produção de substâncias que afetam a integridade das hifas estão entre os mais importantes modos de ação desta levedura

DESEMPENHO NO ENADE EM CIÊNCIAS CONTÁBEIS: ENSINO A DISTÂNCIA (EAD) VERSUS PRESENCIAL

Os últimos anos têm sido marcados por uma evolução no número de matrículas nas instituições de ensino superior, com destaque aos cursos oferecidos na modalidade a distância, os quais atualmente detêm quase 16% de todas as matrículas. Com o objetivo de acompanhar a qualidade da formação ofertada pelas instituições de ensino, o Governo Federal instituiu em 2004 o Sistema Nacional de Avaliação da Educação Superior (SINAES), que por meio do Exame Nacional de Desempenho dos Estudantes (ENADE), avalia o desempenho dos graduandos em função dos conhecimentos adquiridos ao longo da sua formação. Nesse contexto, o trabalho tem por objetivo verificar se existe diferença significativa entre as notas dos alunos dos cursos de Ciências Contábeis das modalidades de ensino a distância e presencial, resultantes do ENADE de 2009, uma vez que as informações sobre a variável modalidade de ensino relativa ao ano de 2012 não foram disponibilizadas nos microdados pelo INEP. A partir de uma amostra de 75.749 estudantes de contabilidade, sendo 5.360 referentes à modalidade de ensino a distância e 70.389 ao ensino presencial, foram aplicados testes de regressão simples e múltipla e o teste não paramétrico de Mann-Whitney para realizar a comparação das notas entre as duas modalidades. Verificou-se que as notas dos alunos dos cursos a distância apresentam-se estatisticamente inferiores às notas dos alunos dos cursos presenciais. Esses resultados sugerem diferenças na qualidade do ensino e a necessidade de acompanhamento do desempenho dos estudantes da modalidade EaD, haja vista a expansão quantitativa que ela vem alcançando no Brasil

Atomistic Study of Water Confined in Silica.

In this work, we have used a combined of atomistic simulation methods to explore the effects of confinement of water molecules between silica surfaces. Firstly, the mechanical properties of water severe confined (~3A) between two silica alpha-quartz was determined based on first principles calculations within the density functional theory (DFT). Simulated annealing methods were employed due to the complex potential energry surface, and the difficulties to avoid local minima. Our results suggest that much of the stiffness of the material (46%) remains, even after the insertion of a water monolayer in the silica. Secondly, in order to access typical time scales for confined systems, classical molecular dynamics was used to determine the dynamical properties of water confined in silica cylindrical pores, with diameters varying from 10 to 40A. in this case we have varied the passivation of the silica surface, from 13% to 100% of SiOH, and the other terminations being SiOH2 and SiOH3, the distribution of the different terminations was obtained with a Monte Carlo simulation. The simulations indicates a lowering of the diffusion coefficientes as the diameter decreases, due to the structuration of hydrogen bonds of water molecules; we have also obtained the density profiles of the confined water and the interfacial tension

CO2 Adsorption Enhanced by Tuning the Layer Charge in a Clay Mineral

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