Melting and crystallization in Ni nanoclusters: The mesoscale regime

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms (5.7 nm) using molecular dynamics with the quantum-Sutten–Chen many-body force field. We find a transition from cluster or molecular behavior below ~500 atoms to a mesoscale nanocrystal regime (well-defined bulk and surface properties) above ~750 atoms (2.7 nm). We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N = Tm,bulk–alphaN^–1/3, dropping from Tm,bulk = 1760 K to Tm,336 = 980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N>400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character

Control based power smoothing for aggregated vertical axis wind turbines

Recently there has been renewed interest in the potential of vertical axis wind turbines (VAWTs), particularly, at very large scale of 10 MW or above owing to their structural simplicity. A significant disadvantage for many types of VAWTs is that the generated power is inherently periodic in nature. This power quality issue is exacerbated as the turbines increase in size and are aggregated requiring the transmission system to withstand large transients. These periodic perturbations in power can be smoothed out by permitting the rotor speed to vary; however, operation in this manner results in a poor capacity factor. A new approach to smoothing the power fluctuations from aggregated VAWTs is proposed, whereby a controller for a small group of turbines is used to adjust the relative phase of the periodic power output from individual machines while maintaining the overall performance of the turbines in a group. Simulation of this control scheme demonstrates that the fluctuations in the aggregated power can be significantly reduced without affecting the mean aggregated power output. The control strategy has been tested by simulation for a range of farm configurations at various wind speeds. The results indicate that the proposed control scheme becomes more effective for increased number of turbines

Impacts of Covid-19 mode shift on road traffic

This article is driven by the following question: as the communities reopen after the COVID-19 pandemic, will changing transportation mode share lead to worse traffic than before? This question could be critical especially if many people rush to single occupancy vehicles. To this end, we estimate how congestion will increases as the number of cars increase on the road, and identify the most sensitive cites to drop in transit usage. Travel time and mode share data from the American Community Survey of the US Census Bureau, for metro areas across the US. A BPR model is used to relate average travel times to the estimated number of commuters traveling by car. We then evaluate increased vehicle volumes on the road if different portions of transit and car pool users switch to single-occupancy vehicles, and report the resulting travel time from the BPR model. The scenarios predict that cities with large transit ridership are at risk for extreme traffic unless transit systems can resume safe, high throughput operations quickly.Comment: 14 pages, 11 figure

Molecular-dynamics simulations of glass formation and crystallization in binary liquid metals: Cu-Ag and Cu-Ni

We used molecular dynamics (MD) to obtain an atomistic description of the melting, glass formation, and crystallization processes in metal alloys. These studies use the quantum Sutton-Chen many-body potentials for Cu, Ni, and Ag to examine the Cu4Ag6 and CuNi alloys. Using cooling rates in the range of 2×10^12 to 4×10^14 K/s, we find that CuNi and pure Cu always form a face-centered-cubic (fcc) crystal while Cu4Ag6 always forms a glass (with Tg decreasing as the quench rate increases). The crystal formers have radius ratios of 1.025 (CuNi) and 1.00 (Cu) while the glass former (CuAg) has a ratio of 1.13, confirming the role of size mismatch in biasing toward glass formation

Strain Rate Induced Amorphization in Metallic Nanowires

Using molecular dynamics simulations with a many-body force field, we studied the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K. For all strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, we find that for both systems the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order and a near vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction. This amorphization which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities exhibits a new mode of amorphization

Friction anisotropy at Ni(100)/(100) interfaces: Molecular dynamics studies

The friction of surfaces moving relative to each other must derive from the atomic interaction at interfaces. However, recent experiments bring into question the fundamental understanding of this phenomenon. The analytic theories predict that most perfect clean incommensurate interfaces would produce no static friction, whereas commensurate aligned surfaces would have very high friction. In contrast recent experiments show that the static friction coefficient between clean but 45° misoriented Ni(001) surfaces is only a factor of 4 smaller than for the aligned surfaces (θ∼0°) and clearly does not vanish (θ is defined as the rotation angle between the relative crystallographic orientations of two parallel surfaces). To understand this friction anisotropy and the difference between analytic theory and experiment, we carried out a series of nonequilibrium molecular dynamics simulations at 300 K for sliding of Ni(001)/Ni(001) interfaces under a constant shear force. Our molecular dynamics calculations on interfaces with the top layer roughed (and rms roughness of 0.8 Å) lead to the static frictional coefficients in good agreement with the corresponding experimental data. On the other hand, perfect smooth surfaces (rms roughness of 0 Å) lead to a factor of 34–330 decreasing of static friction coefficients for misaligned surfaces, a result more consistent with the analytic theories. This shows that the major source of the discrepancy is that small amounts of roughness dramatically increase the friction on incommensurate surfaces, so that misaligned directions are comparable to aligned directions

The trace reconstruction problem for spider graphs

We study the trace reconstruction problem for spider graphs. Let $n$ be the number of nodes of a spider and $d$ be the length of each leg, and suppose that we are given independent traces of the spider from a deletion channel in which each non-root node is deleted with probability $q$. This is a natural generalization of the string trace reconstruction problem in theoretical computer science, which corresponds to the special case where the spider has one leg. In the regime where $d\ge \log_{1/q}(n)$, the problem can be reduced to the vanilla string trace reconstruction problem. We thus study the more interesting regime $d\le \log_{1/q}(n)$, in which entire legs of the spider are deleted with non-negligible probability. We describe an algorithm that reconstructs spiders with high probability using $\exp\left(\mathcal{O}\left(\frac{(nq^d)^{1/3}}{d^{1/3}}(\log n)^{2/3}\right)\right)$ traces. Our algorithm works for all deletion probabilities $q\in(0,1)$.Comment: 17 pages, 3 figure

Atomic simulations of kinetic friction and its velocity dependence at Al/Al and alpha-Al_2O_3/alpha-Al_2O_3 interfaces

Kinetic friction during dry sliding along atomistic-scale Al(001)/Al(001) and alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces has been investigated using molecular dynamics (MD) with recently developed Reactive Force Fields (ReaxFF). It is of interest to determine if kinetic friction variations predicted with MD follow the macroscopic-scale friction laws known as Coulomb's law (for dry sliding) and Stokes' friction law (for lubricated sliding) over a wide range of sliding velocities. The effects of interfacial commensuration and roughness on kinetic friction have been studied. It is found that kinetic friction during sliding at commensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces exceeds that due to sliding at an incommensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interface. For both interfaces, kinetic friction at lower sliding velocities deviates minimally from Coulombic friction, whereas at higher sliding velocities, kinetic friction follows a viscous behavior with sliding damped by thermal phonons. For atomically smooth Al(001)/Al(001), only viscous friction is observed. Surface roughness tends to increase kinetic friction, and adhesive transfer causes kinetic friction to increase more rapidly at higher sliding velocities