Effect of Environment on Microstructure Evolution and Friction of Au–Ni Multilayers

We present results from a systematic investigation of environmental effects on the frictional behavior of Au–Ni multilayer films of varying interlayer spacing. The current results, sliding against ruby spheres in a dry N2 atmosphere, are compared to prior work on the tribological behavior of these materials under ultra-high vacuum (UHV) (Cihan et al. in Sci Rep 9:1–10, 2019). Under both conditions, there is a regime of high friction when the interlayer spacing is large and a regime of low friction when the spacing is small. The low friction regime is associated with a critical grain size below which grain bound-ary sliding is expected to be the dominant mechanism of deformation. A shear-induced alloy formation (60–65 at.% Ni in Au) and a concomitant low friction coefficient was observed with multilayer spacings of 20 nm and lower under UHV. A distinct microstructure was found in dry N2, and is attributed to different interfacial characteristics due to adsorbed species; rather than mixing between Au and Ni layers, only the uppermost Au layers were affected by shearing. These observations are coupled with the friction and wear behavior of multilayer samples sliding under different environments

Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields

We apply Density Functional Theory (DFT) and the DFT+U technique to study the adsorption of transition metal porphine molecules on atomistically flat Au(111) surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional correctly predict the palladium porphine (PdP) low-spin ground state. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with experiments on substituted porphyrins. It exhibits no covalent bonding to Au(111), and the binding energy is a small fraction of an eV. The DFT+U technique, parameterized to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state. Its interaction with the gold substrate is stronger and more site-specific than PdP. The binding can be partially reversed by applying an electric potential, which leads to significant changes in the electronic and magnetic properties of adsorbed MnP, and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine macrocycle. We conjecture that this DFT+U approach may be a useful general method for modeling first row transition metal ion complexes in a condensed-matter setting.Comment: 14 pages, 6 figure

Forces between functionalized silica nanoparticles in solution

To prevent the flocculation and phase separation of nanoparticles in solution, nanoparticles are often functionalized with short chain surfactants. Here we present fully-atomistic molecular dynamics simulations which characterize how these functional coatings affect the interactions between nanoparticles and with the surrounding solvent. For 5 nm diameter silica nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we determined the hydrodynamic drag on two approaching nanoparticles moving through solvent and on a single nanoparticle as it approaches a planar surface. In most circumstances, acroscale fluid theory accurately predicts the drag on these nano-scale particles. Good agreement is seen with Brenner's analytical solutions for wall separations larger than the soft nanoparticle radius. For two approaching coated nanoparticles, the solvent-mediated (velocity-independent) and lubrication (velocity-dependent) forces are purely repulsive and do not exhibit force oscillations that are typical of uncoated rigid spheres.Comment: 4 pages, 3 fig

Modeling local chemistry in the presence of collective phenomena.

Confinement within the nanoscale pores of a zeolite strongly modifies the behavior of small molecules. Typical of many such interesting and important problems, realistic modeling of this phenomena requires simultaneously capturing the detailed behavior of chemical bonds and the possibility of collective dynamics occurring in a complex unit cell (672 atoms in the case of Zeolite-4A). Classical simulations alone cannot reliably model the breaking and formation of chemical bonds, while quantum methods alone are incapable of treating the extended length and time scales characteristic of complex dynamics. We have developed a robust and efficient model in which a small region treated with the Kohn-Sham density functional theory is embedded within a larger system represented with classical potentials. This model has been applied in concert with first-principles electronic structure calculations and classical molecular dynamics and Monte Carlo simulations to study the behavior of water, ammonia, the hydroxide ion, and the ammonium ion in Zeolite-4a. Understanding this behavior is important to the predictive modeling of the aging of Zeolite-based desiccants. In particular, we have studied the absorption of these molecules, interactions between water and the ammonium ion, and reactions between the hydroxide ion and the zeolite cage. We have shown that interactions with the extended Zeolite cage strongly modifies these local chemical phenomena, and thereby we have proven out hypothesis that capturing both local chemistry and collective phenomena is essential to realistic modeling of this system. Based on our results, we have been able to identify two possible mechanisms for the aging of Zeolite-based desiccants

Evolutionary complexity for protection of critical assets.

This report summarizes the work performed as part of a one-year LDRD project, 'Evolutionary Complexity for Protection of Critical Assets.' A brief introduction is given to the topics of genetic algorithms and genetic programming, followed by a discussion of relevant results obtained during the project's research, and finally the conclusions drawn from those results. The focus is on using genetic programming to evolve solutions for relatively simple algebraic equations as a prototype application for evolving complexity in computer codes. The results were obtained using the lil-gp genetic program, a C code for evolving solutions to user-defined problems and functions. These results suggest that genetic programs are not well-suited to evolving complexity for critical asset protection because they cannot efficiently evolve solutions to complex problems, and introduce unacceptable performance penalties into solutions for simple ones

COMP-Angiopoietin-1 Recovers Molecular Biomarkers of Neuropathy and Improves Vascularisation in Sciatic Nerve of ob/ob Mice

mice. mice displayed regeneration of small-diameter endoneural microvessels. Effects of COMP-Ang-1 corresponded to increased phosphorylation of Akt and p38 MAPK upon Tie-2 receptor. mice suggesting COMP-Ang-1 as novel treatment option to improve morphologic and protein expression changes associated with diabetic neuropathy

Theory of photoexcitations in conjugated polymers: The effects of Coulomb interactions

Many-body Coulomb interactions make understanding the complete excitation spectra of conjugated polymers a formidable task, and a variety of sophisticated experimental and theoretical techniques have been used in an attempt to elucidate their complicated electronic structure. It is thus crucial to have a intuitive, physical picture in order to interpret the wide range of available experimental data. We present the results of calculations within an exciton basis which allows for a simple pictorial description of all linear and nonlinear excitations in conjugated polymers, and settles a number of longstanding controversies. The exciton basis further allows us to justify the application of single configuration interaction (SCI) techniques to the understanding of nonlinear optical experiments in the low energy region. We show that SCI can give a clear, self-consistent picture of the photoexcitations in poly(para-phenylene vinylene), a conjugated polymer which has attracted much recent interest

Theoretical model for prediction of high-strength metallic glasses

A method for predicting the ideal strength of metallic glasses based on materials properties—without fitting parameters—is presented, and its accuracy demonstrated for multiple alloys. The theoretical basis for these predictions is the stress-activated transformation of short-range ordered atomic structures into flowing amorphous interfaces with the properties of a supercooled liquid. Our theory for pure metals is extended through a regular solution model in which the enthalpy of fusion is mollified by approximate changes in coordination number between the solid and liquid phases. Additional parameters come from empirical material properties such as density, heat of fusion, and melting temperature.This article is published as Argibay, Nicolas, and Michael Chandross. "Theoretical model for prediction of high-strength metallic glasses." Physical Review Materials 6, no. 11 (2022): 115602. DOI: 10.1103/PhysRevMaterials.6.115602. Copyright 2022 The Authors. Attribution 4.0 International (CC BY 4.0). Posted with permission. DOE Contract Number(s): NA0003525; AC02-07CH11358