603 research outputs found
Reactive coarse-grain simulation for advanced material systems
No full textDecember 2024School of EngineeringWith advancements in computational power and algorithms, molecular dynamics (MD) simulations have become an indispensable tool in materials modeling. Well-designed simplified models, such as coarse-graining (CG) of molecular structures and using scalable potentials, enable researchers to explore larger spatial and temporal scales compared to full atomistic MD. In this thesis, we explore the utility of such models adopted very specifically for specific problems. In the first project, we set up a reactive coarse-grain simulation methodology for epoxy nanocomposite systems. In the second project, we explore the capability of a simplified hcp model to explain valuable physics. In the third and ongoing project, we implemented reactive coarse grain to prepare simulated polymer melt systems to understand the complex entanglements and their effect on the mechanical behavior. Glassy thermosetting polymers represent an important class of engineering materials known for their mechanical strength, chemical resistance, and versatility across industries. Epoxy resin, in particular, is highly valued for its exceptional adhesion, chemical and thermal stability, and electrical insulation properties, making it essential in aerospace, automotive, electronics, and structural applications. Traditional thermosetting epoxies are hard due to heavy cross-linking between the chains, however, very brittle at the same time for the same reason. Concurrent stiffening and toughening of thermosetting polymers have been a longstanding problem in material science. Experiments show that strong and stiff inclusions, such as graphene, offer some promising progress but a clear understanding of such processes is still lacking. On top of that experimental results are still mixed when it comes to simultaneously improving the stiffness and toughness of the matrix using graphene and functionalized graphene flakes. We idntified that the strength of the interface between the matrix and the filler plays an important role in determining the mechanical properties of graphene-reinforced epoxy. In this thesis, we employed reactive CG-MD to understand the effect of the interface strength on the elastic and fracture properties. We developed methodologies to model the epoxy crosslinking process using bump-LJ, a simple but reactive pairwise potential, capturing the network structure that defines their stiffness and brittleness. Reactive CG-MD enabled us to capture the brittle fracture of the epoxy resin. We identified that concurrent stiffening and toughening happens for a moderate adhesive strength between the graphene and the matrix. Both weak and strong interfaces are detrimental. Pyroelectric materials are vital in infrared sensors, thermal imaging, energy harvesting, and temperature sensing technologies. Experimental research done by our collaborators indicates that reducing film thickness enhances pyroelectric properties, making them highly effective for high-sensitivity applications. This dimensionality effect increases progressively from van der Waals (vdW) to quasi-vdW to ionic/covalent materials and is hypothesized to result from enhanced electron-phonon coupling, related to the Debye-Waller factor. In this thesis, we developed a simplified hcp structure using the bump LJ potential to simulate atomic vibrations across three material classes with varying out-of-plane bond strengths and at different thickness levels. Our findings show that Debye-Waller factor increases with reduction in membrane thickness and the enhancement is more pronounced in covalently bonded materials than in weaker vdW-bonded materials, supporting the theoretical framework and shedding light on the mechanisms driving this dimensionality effect.Ph
MHD mixed convection analysis of non-Newtonian power law fluid in an open channel with round cavity
No full textMechanical anisotropy and tension-compression asymmetry of Au coated Si nanowafers: An atomistic approach
No full textMechanical Properties of Au Coated Si Nanowafer: an Atomistic Study
No full textCombined gold and silicon nano-system has spurred tremendous interest in the
scientific community due to its application in different metal-semiconductor
electronic devices and solar driven water splitting cells. Silicon, fabricated
on gold layer, is prone to gold atom diffusion at its surface. In this study,
detailed analysis of mechanical properties of gold coated silicon nanowafer is
studied by performing molecular dynamics tensile and compressive simulations.
The effects of temperature, gold coating thickness, strain rate and
crystallographic orientation of silicon on the mechanical properties are
observed for the nanowafer. It is found that both the ultimate tensile and
compressive strength show inverse relationship with temperature. The nanowafer
fails mainly by slipping along {110} plane due to excessive shear when loaded
in [100] direction while a mixed slip and crack type failure occurs for 300K.
Interesting crystallographic transformation from fcc to hcp crystal is observed
in gold layer for the highest gold layer thickness during tension. The effects
of strain rate in tension and compression is also studied. Finally, the crystal
orientation of silicon is varied and the tension-compression asymmetry inn the
gold coated silicon nanowafer is investigated. Reverse tension-compression
asymmetry is observed in case of loading along [110] crystal orientation. The
failure mechanism reveals that interesting crystal transformation of silicon
occurs during compression leading to early yielding of the material.Comment: 24 pages, 14 figure
Defending against Malicious Websites: Themed Threats, Detection, and Law-Enforcement
No full textThe author has granted permission for their work to be available to the general public.Malicious websites have become a main cyber threat. Despite the substantial effort made by researchers and practitioners, some fundamental problems regarding effective defenses against these attacks remain open, such as: What are the emerging trends of malicious websites? How should we cope with the new trends? What do we need to do to help law-enforcement deal with them? This dissertation addresses these problems by making three contributions. The first contribution is to characterize emerging threats of themed malicious websites, which represent one trend as evidenced by the many malicious websites exploiting the COVID incident. The characterization offers a deep understanding of the attacks, which leads to the second contribution, namely the investigation of how to detect the emerging themed malicious websites exploiting the COVID-19 incident. While the preceding two contributions are from a purely technological point of view, the third contribution investigates the gap between technology and law-enforcement with respect to malicious websites. Understanding and addressing the gap is essential because we anticipate that the law-enforcement eventually needs to be involved in dealing with malicious websites, if not already. For this purpose, we focus on investigating how to support the law-enforcement dealing with blacklisted websites while highlighting two important factors: one is the trustworthiness of Machine Learning methods in predicting malicious websites, which is important because blacklists are not perfect; and the other is how to interpret or explain the individual predictions (possibly in the court), which existing black-box ML models can not provide. The resulting methodology could be leveraged to cope with malicious websites towards ultimately eliminating, or at least adequately mitigating, them. Finally, we present case studies with real-world datasets to show the usability and efficacy of the proposed methods.Computer Scienc
Temperature and size effect on the mechanical properties of indium phosphide nanowire: An atomistic study
No full textOPENNESS OF THE COUNTRY, CURRENCY SUBSTITUTION AND MONETARY POLICY
No full textAutor razmatra utjecaj valutne supstitucije na vođenje monetarne politike razlikujući pritom "male" i "velike" zemlje. Koristeći usporednu ekonometrijsku analizu i naglašavajući njezina ograničenja, autor zaključuje da u zemljama s malim brojem stanovnika i valutnom supstitucijom postoji viši omjer međunarodnih pričuva prema rezervnom novcu.The author considers the influence of currency substitution on conducting of monetary policy, distinguishing here "small" and "big" countnes. Small countnes are affected by international capital flovvs and change of rates, thus it is presumed that this fact is reflected in the structure of central banks property. The author expects that in open economies (and that means in economies where currency substitution more easily appears-spread usage of foreign currency in money functions) the share of international assets reserves is higher than in closed economies. Using parallel econometric analysis and emphasizing its restrictions, the author concludes that in countnes with a small number of population and currency substitution there is a higher ratio of international reserves compared to reserve money
Atomistic Investigation on Mechanical Properties of Sn-Ag-Cu Based Nanocrystalline Solder Material
No full textGLOBALIZATION AND POLITICAL STRATEGY
No full textAutor raščlanjuje sporni koncept globalizacije na njezine tehnologijske, političke, kulturalne, ekonomske i društvene aspekte. Pritom polemizira sa S. Huntingtonom, a dijelom i s J. Grayom, pridružujući se argumentima P. Chatterjeea o nacionalističkoj strategiji i politici. Jameson zaključuje da su udruživanje (combination), društveni kolektivitet i globalna regulacija fokusi svakoga strategijski uspješnog odgovora na trendove globalizacije.The author analyses the controversial concept of globalization from its technological, political, cultural, economic and social aspects. He refers to S. Huntington, and also to J. Gray, and uses P. Chaterjee’s arguments on nationalist strategy and politics. Jameson concludes that combination, social collective and global regulation are the at the core of all strategically successful responses to the globalization trends
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