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Molecular Dynamics Simulations of Dodecanethiol Coated Gold Nanoparticles on Organic Liquid ToluenePoddar, Nitun Nirjhar January 2013 (has links)
No description available.
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A Molecular Dynamics Study on Tension Deformation Behavior in Magnesium NanocrystalsXi, Dalei 28 September 2018 (has links)
No description available.
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The Electrical Double Layer at the Water-Silica Interface: Structure, Dynamics, Response to External Fields, and Biomolecules AdsorptionShi, Bobo 01 September 2016 (has links)
No description available.
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Molecular Simulation Investigation on the Structure-Activity Relationships at Inorganic-Biomolecule InterfacesZhao, Weilong 04 October 2016 (has links)
No description available.
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Melting, Solidification and Sintering/Coalescence of NanoparticlesWang, Ningyu 01 November 2010 (has links)
No description available.
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Atomistic Modeling of Defect Energetics and Kinetics at Interfaces and Surfaces in Metals and AlloysAlcocer Seoane, Axel Emanuel 02 January 2024 (has links)
Planar defects such as free surfaces and grain boundaries in metals and alloys play important roles affecting many material properties such as fracture toughness, corrosion resistance, wetting, and catalysis. Their interactions with point defects and solute elements also play critical roles on governing the microstructural evolution and associated property changes in materials. This work seeks to use atomistic modeling to obtain a fundamental understanding of many surface and interface related properties and phenomena, namely: orientation-dependent surface energy of elemental metals and alloys, segregation of solute elements at grain boundaries and their impact on grain boundary cohesive strength, and the controversial sluggish diffusion in both the bulk and grain boundaries of high entropy alloys. First, an analytical formula is derived, which can predict the surface energy of any arbitrary (h k l) crystallographic orientation in both body-centered-cubic (BCC) and face-centered-cubic (FCC) pure metals, using only two or three low-index (e.g., (100), (110), (111)) surface energies as input. This analytical formula is validated against 4357 independent single element surface energies reported in literature or calculated by the present author, and it proves to be highly accurate but easy to use. This formula is then expanded to include the simple-cubic (SC) structure and tested against 4542 surface energies of metallic alloys of different cubic structures, and good agreement is achieved for most cases. Second, the effect of segregation of substitutional solute elements on grain boundary cohesive strength in BCC Fe is studied. It is found that the bulk substitution energy can be used as an effective indicator to predict the embrittlement or strengthening potency induced by the solute segregation at grain boundaries. Third, the controversial vacancy-mediated sluggish diffusion in an equiatomic FeNiCrCoCu FCC high entropy alloy is studied. Many literature studies have postulated that the compositional complexity in high entropy alloys could lead to sluggish diffusion. To test this hypothesis, this work compares the vacancy-mediated self-diffusion in this model high entropy alloy with a hypothetical single-element material (called average-atom material) that has similar average properties as the high entropy alloy but without the compositional complexity. The results show that the self-diffusivities in the two bulk systems are very similar, suggesting that the compositional complexity in the high entropy alloy may not be sufficient to induce sluggish diffusion in bulk high entropy alloys. Based on the knowledge learned from the bulk alloy, the exploration of the possible sluggish diffusion has been extended to grain boundaries, using a similar approach as in the study of self-diffusion in bulk. Interestingly, the results show that sluggish diffusion is evident at a Σ5(210) grain boundary in the high entropy alloy due to the compositional complexity, especially in the low temperature regime, which is different from the bulk diffusion. The underlying mechanisms for the sluggish diffusion at this grain boundary is discussed. / Doctor of Philosophy / Human beings have utilized metals and alloys for over ten millennia and learned much from them. Based on the accumulated knowledge, they have countless applications in our current daily life. However, there is still much to learn for improving our current technology and even opening new opportunities. Throughout most of history, our understanding of these materials was largely obtained through empirical experimentation and refining them into theories and scientific laws. Nowadays, due to the advancements in computer simulations, we can learn more by modeling the behaviors of metals and alloys at the length and time scales that are either be too arduous, costly, or currently impossible experimentally.
This work aims at using computer modeling to study some important surface/interface related physical behaviors and properties in metals and alloys at the atomistic scale. First, this work intends to develop a robust surface energy model in an analytical form for any crystallographic orientation. Surface energy is an important material property for many surface-related processes such as fracturing, wetting, sintering, catalysis, and crystalline particle shape. Surface energy is different at different surface orientations, and predicting this difference is important for understanding these surface phenomena. Second, the effect of solute segregation on grain boundary cohesive strength is studied. Most commonly used metallic materials consist of many small crystalline grains and the borders between them are called grain boundaries, which are weak spots for fracture. The minimum energy required to split a boundary is called the grain boundary cohesive strength. The presence of solutes or impurities at grain boundaries can further alter the cohesive strength. A better understanding of this phenomena will eventually help us develop more fracture-resistant materials. The third project deals with the possible sluggish/retarded diffusion in high entropy alloys, which contain five or more principal alloying elements and have many unique mechanical, radiation-resistant, and corrosion-resistant properties. Many researchers attribute these unique properties to the slow species diffusion in these alloys, but its existence is still controversial. This work studies the atomic-level diffusion mechanisms in an FeNiCrCoCu high entropy alloy both in bulk (grain interior) and at grain boundaries in order to determine if sluggish diffusion is present and its causes.
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Grain Boundary Segregation: the New SproutsBokstein, Boris, Itckovich, Alexei, Pokhvisnev, Yury, Rodin, Alexei 21 September 2022 (has links)
Some aspects of grain boundary segregation (GBS) are discussed. This paper adds two new
sprouts. The first is connected with formation of the atomic complexes in boundary region and their
effect on grain boundary diffusion (GBD). The second – with a nonhomogeneity of energy
distribution between boundary sites.
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The effect of chirality and steric hindrance on intrinsic backbone conformational propensities: tools for protein designChilders, M.C., Towse, Clare-Louise, Daggett, V. 11 May 2016 (has links)
No / The conformational propensities of amino acids are an amalgamation of sequence effects, environmental effects and underlying intrinsic behavior. Many have attempted to investigate neighboring residue effects to aid in our understanding of protein folding and improve structure prediction efforts, especially with respect to difficult to characterize states, such as disordered or unfolded states. Host-guest peptide series are a useful tool in examining the propensities of the amino acids free from the surrounding protein structure. Here, we compare the distributions of the backbone dihedral angles (φ/ψ) of the 20 proteogenic amino acids in two different sequence contexts using the AAXAA and GGXGG host-guest pentapeptide series. We further examine their intrinsic behaviors across three environmental contexts: water at 298 K, water at 498 K, and 8 M urea at 298 K. The GGXGG systems provide the intrinsic amino acid propensities devoid of any conformational context. The alanine residues in the AAXAA series enforce backbone chirality, thereby providing a model of the intrinsic behavior of amino acids in a protein chain. Our results show modest differences in φ/ψ distributions due to the steric constraints of the Ala side chains, the magnitudes of which are dependent on the denaturing conditions. One of the strongest factors modulating φ/ψ distributions was the protonation of titratable side chains, and the largest differences observed were in the amino acid propensities for the rarely sampled αL region. / NIH
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Size Effect in Polymeric Materials: the Origins and the Multi-physics Responses in Ultrasound FieldsPeng, Kaiyuan 06 January 2021 (has links)
The size effect in the thermo-mechanical behavior of polymeric materials is a critically important
phenomenon and has been the subject of many researches in past decades. For example,
polystyrene (PS), a widely used polymeric material, is brittle at the bulk state. When the
dimensions decreases to the nanoscale, such as PS in nanofibers, their ductility becomes
orders higher than their bulk state. In recent years a number of diverse applications, such
as scaffolds in tissue engineering, drug delivery devices, as well as soft robotics, are designed
by utilizing the unique properties of polymers at nanoscale. However, the inside mechanism
of the size dependency in polymeric materials are still not clear yet. In this dissertation, systematic
computational and experimental studies are made in order to understand the origins
of the size effect for one- and two-dimensional polymeric materials. This framework is also
expanded to investigate the size-dependent multi-physics response of functional polymeric
materials (shape memory polymers) which are actuated by high-intensity focused ultrasound
(HIFU). Our computational studies are based on molecular dynamic (MD) simulations at
the atomistic scale, and experimentally-validated finite element models at the bulk level.
From bottom-up direction, molecular dynamics can reveal the mechanisms of the size effect
in polymers at molecular level, and help predict properties of the bulk materials. In this
research, MD simulations are performed to track the origins of the size-effect in the mechanical
properties of PE and PS nanofibers. In addition, the size-dependent thermal response
of functional polymeric films is also studied at the atomistic scale by utilizing molecular dynamics simulations to predict the thermal properties and actuation mechanisms in these
materials when subjected to HIFU fields. From top-down direction, experiments and finite
element analysis, are also conducted in this research. An experimentally-validated finite
element framework is built to study the mechanical response of shape memory polymers
(SMPs) triggered by HIFU. As an external trail towards application fields, a SMP composite
with enhanced shape memory ability and also a two-way SMP are synthesized. A smart
gripper and also a self-rolling structure are designed by using these SMPs, which approves
that these SMPs are good components in designing soft robotics. Finally, The influence of
evaporation during fiber forming process is investigated by molecular dynamics simulation.
It is found that the formation of the microstructure of polymeric fibers at nanoscale depends
on the balance of stretching force and evaporation rate when the fiber is forming. / Doctor of Philosophy / Thermomechanical properties of a thin fiber, a thin film and a cube made of a polymer are
significantly different. Although, based on the extensive research that has been performed in
recent years our understanding of this size-dependency is advanced to a great degree in the
past decades, there are still many unanswered basic questions that can only be addressed
by performing computational and experimental investigation at different length scales, from
atomistic up to bulk level in polymers. In this research we target exploring some unknown aspects
of the size dependency in the thermomechanical properties of polymers by investigating
their deformation mechanisms at different length scales. As the first step, we will investigate
the mechanical properties of polymeric fibers. For these fibers, the mechanical properties
are strongly connected to the fiber's diameter. The prevailing hypothesis is that this size
dependency is closely related to the thickness of the surface layer of the nanofibers. Our
results show some unknown origins behind the size dependency of the mechanical properties
in polyethylene (PE) and polystyrene (PS) nanofibers, which originate from the deformation
mechanisms at the atomistic scale. In addition, not just the mechanical properties, the
thermal properties and response of functional polymers subjected to an external stimulation
are also related to their size. For example, the thermal conductivity of a fiber, a sheet and a
cube may be significantly different. Our study shows the thermal responses of different polymers
triggered by ultrasound are also different. The size and the type of the polymers will
both have influence on the final temperature in the polymeric materials, when the polymeric materials are heated by same ultrasound source. We also have applied our computational
and experimental frameworks to investigate this phenomenon. In addition, we also used a
new shape memory polymer composite and a two-way shape memory polymer on designing
soft robotics-like structures. Overall this research indicates that both mechanical response
and thermal responses of polymers are highly related to their dimension. Taking advantage
of these unique size effects, and by tailoring this property, diverse devices can be made for
being used in a broad range of applications.
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Separation and Properties of La₂O₃ in Molten LiF-NaF-KF SaltYang, Qiufeng 21 December 2018 (has links)
Studies on nuclear technology have been ongoing since nuclear power became uniquely important to meet climate change goals while phasing out fossil fuels. Research on the fluoride salt cooled high temperature reactor (FHR), which is funded by the United States Department of Energy (DOE), has developed smoothly with the ultimate goal of a 2030 deployment. One challenge presented by FHR is that the primary coolant salt can acquire contamination from fuel failure and moisture leaking into the system. If contamination happens, it will result in a low concentration of fission products, fuel, transuranic materials and oxide impurities in the coolant. These impurities will then affect the properties of the molten salt in the long term and need to be removed without introducing new impurities. Most of the research conducted recently has focused on impurity separation in chloride molten salts. More research urgently needs to be conducted to study the impurity separation method for the fluoride molten salts.
In this study, the La₂O₃-LiF-NaF-KF (La₂O₃-FLiNaK) system is used to demonstrate impurity separation in molten fluoride salt. Since lanthanum oxide needs to be dissolved in the fluoride molten salt and studies in this field are still not complete, the solubility of lanthanum oxide in FLiNaK have been measured at different temperatures to obtain the temperature-dependent solubility and understand the corresponding dissolution mechanisms first. In the solubility related experiments, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is utilized to analyze the concentration of lanthanum ions in the molten FLiNaK salt, while X-ray powder diffraction (XRD) was applied to determine the phase patterns of molten salt. Second, electrochemical experiments with tungsten and graphite as working electrodes were conducted individually to demonstrate the separation of the dissolved oxide from the salt. When the tungsten working electrode was applied, the lanthanum ions were reduced to lanthanum metal at the tungsten cathode, while the fluorine ions reacted with the tungsten anode to form tungsten fluoride. In the experiments, the production of tungsten fluoride could lead to increasing current in the cell, even overload. Moreover, theoretically, tungsten fluoride WF4 is soluble in the fluoride salt thus introducing new impurities. All these issues make tungsten not the best choice when applied to the separation of oxygen ions. Therefore, another common working electrode graphite is used. It not only has all the advantages of tungsten, but also has good performance on separation of oxygen ions. When the graphite electrode was applied, the lanthanum ions were separated in the form of lanthanum carbide (LaC₂), while the oxygen ions can be removed in the form of carbon dioxide (CO₂) or carbon monoxide (CO). In addition, only graphite was consumed during the whole separation process, which is why the graphite anode electrode is called the “sacrificial electrode”. Third, First Principle Molecular Dynamics (FPMD) simulations with Vienne Ab initio Simulation Package (VASP) was conducted to study the properties of the fluoride molten salt. In this study, the structure information and enthalpy of formation were obtained. Generally, the simulation process can be divided into four steps: (1) the simulation systems are prepared by packing ions randomly via Packmol package in the simulation cell; (2) an equilibrium calculation is performed to pre-equilibrate the systems; (3) FPMD simulations in an NVT ensemble are implemented in VASP; (4) based on the FPMD simulations results, the first peak radius and the first-shell coordination number were evaluated with partial radial distribution function (PRDF) analysis to determine the statistics of molten salt structure information, while the transport properties, e.g., the self-diffusion coefficient was calculated according to the function of mean square displacement (MSD) of time generated by the Einstein-Smoluchowshi equation. The viscosity and ionic conductivity were obtained by combining the self-distribution coefficient with the Einstein-Stokes formula and Nernst-Einstein equation. / Master of Science / With the fast development of modern society and economy, more and more energy is urgently needed to meet the growth of industry. Since the traditional energy, such as nature gas, coal, has limited storage and not sustainable, nuclear energy has attracted much attention in the past few decades. Although lots of study has been conducted by thousands of researchers which has attributed to application of nuclear power, there are still some concerns in this field, among which, impurities removal is the most difficult part.
Fluoride salt cooled high temperature reactor (FHR) is one of the most promising Gen IV reactor types. As the name indicates, molten salt is the coolant to serve as the heat exchanger intermedium. In addition, it’s inevitable that fission products, i.e. lanthanum, moisture, would leak into the coolant pipe, thus affect the molten salt properties, even degrade reactor performance, therefore, those impurities must be removed without introducing new impurities.
In this study, the La₂O₃-LiF-NaF-KF (La₂O₃-FLiNaK) system is used to demonstrate impurity separation into molten fluoride salt. First, solubility of lanthanum oxide in FLiNaK has been measured at different temperatures to understand its dissolution mechanisms. Then, electrochemical experiments with tungsten and graphite as working electrodes were conducted individually to demonstrate the separation of the dissolved oxide from the salt. It has been concluded that tungsten performed well to separate La3+, while failed in the separation of O2-. However, graphite working electrode has succeeded in the removal of La³⁺ and O²⁻. Finally, molecular dynamic simulation with first principle was also conducted to further understand the local structure and heat of formation in the molten FLiNaK and La₂O₃-FLiNaK salt.
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