A computational study of diffusion in a glass-forming metallic liquid

Investigation of structural evolution in the Cu-Zr metallic glass at cryogenic temperatures by using molecular dynamics simulations

In the present work, investigation of structural evolution of Cu₃₃Zr₆₇ specimen during the cooling process from 2500 down to the 300 K, 200 K, 150 K, 100 K, 50 K, and 10 K has been performed at cooling rate of 5 K/ps using molecular dynamics simulation. The pair distribution function (PDF) reveals that Zr‒Zr pair causes the splitting of the first peak of the Cu₃₃Zr₆₇ glass at a lower temperature with an increase in height. Splitting of the first and second peaks supports the presence of the inhomogeneous structure with a statistical average of crystal-like and disordered structural regions in the Cu₃₃Zr₆₇ glass. Voronoi cluster analysis indicated that quasi icosahedral clusters such as < 284 > , < 0285 > , and < 0282 > ; mixed-type cluster such as < 0364 > ; and crystal-like clusters such as < 0446 > are responsible for stabilization of glassy phase at 300 K, 200 K, 150 K, 100 K, 50 K, and 10 K. Similarly, the maximum population of the Cu-centered and Zr-centered < 0286 > quasi icosahedral clusters support the stability of the glassy phase over the studied temperature range. Besides, the maximum population of Cu-centered < 0367 > and Zr-centered < 0364 > , < 0367 > , < 0363 > , and < 0365 > mixed-type clusters and Cu-centered < 0448 > and Zr-centered < 0448 > , < 0445 > , < 0446 > , and < 0444 > crystal-like clusters support the possibility of the presence of intermediate phase of CuZr₂ at lower temperatures as observed from PDFs. Mean square displacement (MSD) for the Cu₃₃Zr₆₇ glass shows that the diffusion coefficient of Cu and Zr atoms reduces with decreasing temperature from 300 to 10 K. Diversity parameter (d) was found to decrease with decreasing temperature.

Relations Between Dynamic Localization and Solute Diffusion in Polymers

Various public health concerns can arise from the unintended leaching of additives and impurities from polymeric medical devices or food packaging, which is directly related to each solute's diffusivity D. Both experimental and simulation methods can be used to quantify D, but slow diffusion at physiologic temperature in glassy polymers can render these approaches impractical. Here, we investigate a simulation approach with the potential to more rapidly calculate D. Specifically, we examine links between dynamic localization, characterized by the Debye-Waller factor, ⟨u²⟩, and D in a variety of polymer/solute systems using atomistic molecular dynamics (MD) simulations. Using short, high-temperature MD simulations to estimate D at physiologic temperature, we find that the relation ln D ∝ 1/⟨u²⟩ quantitatively predicts D for small solutes and produces an upper-bound estimate of D for larger solutes. Upper-bound estimates are useful in certain contexts, and we compare our results with another approach for determining upper bounds, the Piringer model, to show where each method may be useful. Then, we examine a modified relation where the Debye-Waller factor is rescaled by the mode coupling temperature T_c, which can produce better estimates of D if T_c is carefully chosen. Last, we compare our approach with several other models that relate temperature or localized dynamics with diffusivity. Although each of these approaches can be used to model D across wide temperature ranges using one or more adjustable parameters, none of them are truly predictive in glassy polymers. Further developments are needed to predict the optimal values of the adjustable parameters a priori.

Hydrogen Storage in Propane-Hydrate: Theoretical and Experimental Study

There have been studies on gas-phase promoter facilitation of H2-containing clathrates. In the present study, non-equilibrium molecular dynamics (NEMD) simulations were conducted to analyse hydrogen release and uptake from/into propane planar clathrate surfaces at 180–273 K. The kinetics of the formation of propane hydrate as the host for hydrogen as well as hydrogen uptake into this framework was investigated experimentally using a fixed-bed reactor. The experimental hydrogen storage capacity propane hydrate was found to be around 1.04 wt% in compare with the theoretical expected 1.13 wt% storage capacity of propane hydrate. As a result, we advocate some limitation of gas-dispersion (fixed-bed) reactors such as the possibility of having un-reacted water as well as limited diffusion of hydrogen in the bulk hydrate.

Composition- and temperature-dependent liquid structures in Al-Cu alloys: an ab initio molecular dynamics and x-ray diffraction study

The composition- and temperature-dependent liquid structures in eight Al_rich-Cu binary alloys (from hypoeutectic Al₉₃Cu₇ to hypereutectic Al₇₀Cu₃₀) have been experimentally and computationally studied by x-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. The remarkable agreements of structure factors for all liquid Al_rich-Cu alloys obtained from high-temperature high-energy XRD measurements and AIMD simulations have been achieved, which consolidates the analyses of structural evolutions in Al_rich-Cu liquids during the cooling processing by AIMD simulations. The heat capacity of liquid Al_rich-Cu alloys continuously increases and presents no abnormal peak when reducing the temperature, which differs from the reported prediction for 55-atom Al_rich-Cu nanoliquids. The diffusivities of Al and Cu undergo an increasing deviation from Arrhenius behavior by tuning Cu concentration from 7 to 30 atomic percentages, correlated to the local ordering in these liquids by means of coordination number, bond-angle distribution, Honeycutt-Andersen index, bond-orientational order and Voronoi tessellation analyses. Upon cooling, the microstructure of the liquid Al_rich-Cu alloys inclines to form Al₂Cu crystal-like local atomic ordering, especially in the hypereutectic liquids. The favorable short-range ordering between Cu and Al atoms could cause the non-Arrhenius diffusion behavior.