arXiv subjects

R. Ashcraft

Publications and source records attributed to R. Ashcraft.

5 recordsLinked to original sources

arXiv2019

Experimental measurements of the temperature-dependent Van Hove function in a $\text{Zr}_{80} \text{Pt}_{20}$ liquid

Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique the time-dependent pair-distribution function (i.e. the Van Hove function) has been determined for liquid Zr80Pt20. We show that the decay-time of the first peak of the Van Hove function is directly related to the Maxwell relaxation time of the liquid, which is proportional to the shear viscosity. This result demonstrates that the local dynamics for increasing or decreasing the coordination number of local clusters by one determines the viscosity at high temperature, supporting earlier predictions from molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2019

Prediction of glass formability from liquid properties

Glass formation is one of the most interesting phenomena in the condensed matter field. Considerable effort has gone into understanding and predicting the glass formability. However, the previous prediction requires the glass first made before the prediction can be performed. Here, we propose a new prediction formula using liquid properties only. Moreover, we demonstrated that the similarity between liquid and crystalline structure plays an important role in determine the glass formability. Previously, only the kinetics of nucleation and growth processes have been considered.

cond-mat.mtrl-sci↗

arXiv2018

Estimates of bond length and thermal expansion coefficients from x-ray scattering experimental data using reverse Monte Carlo simulations

The previously discussed anomalous behavior (i.e. negative) of the thermal expansion coefficient obtained from the pair correlation function is examined in the context of the nearest-neighbor distance (bond length) distribution. The bond length distribution is obtained from a Voronoi tessellation analysis of the atomic structures obtained from both reverse Monte Carlo simulations of x-ray scattering data and molecular dynamics simulations. When a robust measure of central tendency (mean or median) is used a positive thermal expansion is obtained from the temperature-dependent bond length that has the same magnitude as that obtained from direct measurements of the volume as a function of temperature. The same is true when larger neighbor distances, as obtained in higher order peaks in the pair distribution function are tracked. This calls into question the recent claim that fragility of metallic liquids is embedded in these higher order peaks. It also shows that the previously reported anomalous contraction of the bond length arise from tracking the mode, which does not account for the skewness of the distribution.

cond-mat.mtrl-sci↗

arXiv2018

Assessing the Reliability of Minimally Constrained Reverse Monte Carlo Simulations for Model Metallic Liquids

Molecular dynamics simulations using semi-empirical potentials are examined for three liquids to check the reliability of reverse Monte Carlo (RMC) simulations to reproduce atomic configurations when only total pair correlation functions (TPCF) are used as constraints. The local structures are determined from a Voronoi tessellation of the ensemble and compared with the structures obtained by RMC in terms of asphericity, volume, coordination number, Voronoi index, and nearest-neighbor distance. It is found that in general the distributions generated from RMC do not match the MD configurations, using the $L^1$ (taxicab) distance as a metric, although in some cases a measure of central tendency for the distribution did match. Since only TPCFs are typically used to constrain the RMC simulations of experimental data, this study establishes the limits on what can be learned by this analysis. It indicates that caution should be used when interpreting RMC-generated structures using few constraints since many structural quantities are not reproduced well.

cond-mat.mtrl-sci↗

arXiv2017

Correlation of the fragility of metallic liquids with the high temperature structure, volume, and cohesive energy

The thermal expansion coefficients, structure factors, and viscosities of twenty-five equilibrium and supercooled metallic liquids have been measured using an electrostatic levitation (ESL) facility. The structure factor was measured at the Advanced Photon Source, Argonne, using the ESL. A clear connection between liquid fragility and structural and volumetric changes at high temperatures is established; the observed changes are larger for the more fragile liquids. It is also demonstrated that the fragility of metallic liquids is determined to a large extent by the cohesive energy and is, therefore, predictable. These results are expected to provide useful guidance in the future design of metallic glasses.

cond-mat.soft↗