arXiv subjects

K. F. Kelton

Publications and source records attributed to K. F. Kelton.

At least 19 recordsLinked to original sources

arXiv2023

Crystal Nucleation in an AlNiZr Metallic Liquid: Within and Beyond Classical Nucleation Theory

The Classical Nucleation Theory (CNT) has played a key role in crystal nucleation studies since the 19th century and has significantly advanced the understanding of nucleation. However, certain key assumptions of CNT, such as a compact and spherical nucleating cluster and the concept of individual diffusive jumps are questionable. The results of molecular dynamics (MD) studies of crystal nucleation in a Al20Ni60Zr20 metallic liquid demonstrate that the nucleating cluster is neither spherical nor compact. The seeding method was employed to determine the critical cluster size and nucleation parameters from CNT, which were then compared to those derived from the Mean First Passage Time (MFPT) method. While the CNT-based nucleation rate aligns well with experimental data from similar metallic liquids, the MFPT rate differs significantly. Further, contrary to the assumption of individual jumps for atoms to join the nucleating cluster, a cooperative mechanism of attachment or detachment is observed. This is accompanied by synchronized changes in the local potential energy. Similar cooperative motion also appeared in a non-classical nucleation process, particularly during the coalescence of nuclei.

cond-mat.mtrl-sci↗

arXiv2022

Crystal Nucleation and Growth in Liquids: Cooperative Atom Attachment and Detachment

Classical theories of crystal nucleation and growth from the liquid assume activated processes that are interface limited, with the atoms individually joining the growing interface by jumps that occur at a rate that is determined by the diffusion coefficient in the liquid phase. These assumptions are in contradiction with the results of molecular dynamics studies that are presented here for supercooled Ni and Al20Ni60Zr20. Instead of diffusion-based attachment across the interface, atoms join the interface by making small changes so as to match the orientational order parameter of the nucleating crystal. Further, instead of joining individually multiple atoms join cooperatively, with the number of cooperative atoms increasing with decreasing temperature.

physics.atom-ph↗

arXiv2022

Deviations from Arrhenius dynamics in high temperature liquids, a possible collapse, and a viscosity bound

Liquids realize a highly complex state of matter in which strong competing kinetic and interaction effects come to life. As such, liquids are more challenging to understand than either gases or solids generally. In weakly interacting gases, the kinetic effects dominate. By contrast, low temperature solids typically feature far smaller fluctuations about their ground state. Notwithstanding their complexity, with the exception of quantum fluids and supercooled liquids, various aspects of common liquid dynamics such as their dynamic viscosity are often assumed to be given by rather simple, Arrhenius-type, activated forms with nearly constant (i.e., temperature independent) energy barriers. In this work, we analyze experimentally measured viscosities of numerous liquids far above their equilibrium melting temperature to see how well this assumption fares. We find, for the investigated liquids, marked deviations from simple activated dynamics. Even far above their equilibrium melting temperatures, as the temperature drops, the viscosity of these liquids increases more strongly than predicted by activated dynamics dominated by a single uniform energy barrier. For metallic fluids, the scale of the prefactors of the best Arrhenius fits for the viscosity is typically consistent with that given by the product $nh$ with $n$ the number density and $h$ Planck's constant. In various fluids that we examined, $nh$ constitutes a lower bound scale on the viscosity. We find that a scaling of the temperature axis (complementing that of the viscosity) leads to a partial collapse of the temperature dependent viscosities of different fluids; such a scaling allows for a functional dependence of the viscosity on temperature that includes yet is far more general than activated Arrhenius form alone. We speculate on relations between non-Arrhenius dynamics and thermodynamic observables.

cond-mat.soft↗

arXiv2021

Demonstration of the effect of stirring on nucleation from experiments on the International Space Station using the ISS-EML facility

The effect of fluid flow on crystal nucleation in supercooled liquids is not well understood. The variable density and temperature gradients in the liquid make it difficult to study this under terrestrial gravity conditions. Nucleation experiments were therefore made in a microgravity environment using the Electromagnetic Levitation facility on the International Space Station on a bulk glass-forming Zr57Cu15.4Ni12.6Al10Nb5 (Vit106), as well as Cu50Zr50 and the quasicrystal-forming Ti39.5Zr39.5Ni21 liquids. The maximum supercooling temperatures for each alloy were measured as a function of controlled stirring by applying various combinations of radio frequency positioner and heater voltages to the water-cooled copper coils. The flow patterns were simulated from the known parameters for the coil and the levitated samples. The maximum nucleation temperatures increased systematically with increased fluid flow in the liquids for Vit106, but stayed nearly unchanged for the other two. These results are consistent with the predictions from the coupled-flux model for nucleation.

cond-mat.soft↗

arXiv2020

Nucleation Pathways in Barium Silicate Glasses

Nucleation is generally viewed as a structural fluctuation that passes a critical size to eventually become a stable emerging new phase. However, this concept leaves out many details, such as changes in cluster composition and competing pathways to the new phase. In this work, both experimental and computer modeling studies are used to understand the cluster composition and pathways. Monte Carlo and molecular dynamics approaches are used to analyze the thermodynamic and kinetic contributions to the nucleation landscape in barium silicate glasses. Experimental techniques examine the resulting polycrystals that form. Both the modeling and experimental data indicate that a silica rich core plays a dominant role in the nucleation process.

cond-mat.mtrl-sci↗

arXiv2020

The Low-Temperature Nucleation Rate Anomaly in Silicate Glasses is an Artifact

Over the past 40 years measurements of the nucleation rates in a large number of silicate glasses have indicated a breakdown in the widely used Classical Nucleation Theory (CNT) for temperatures below that of the peak nucleation rate. The data show that instead of steadily decreasing with decreasing temperature, the work of critical cluster formation enters a plateau, and even starts to increase. While many explanations have been offered to explain this anomaly, none have provided a satisfactory answer. We present the first experimental results that demonstrate that the anomaly is not real, but is instead an artifact arising from an insufficient annealing time at the low temperatures. The time-dependent nucleation rate was measured in a 5BaO.8SiO2 glass at a temperature 50 K below the peak nucleation rate temperature for a time many times longer than any previous measurement time for a silicate glass. The new data give results that are consistent with the predictions of the CNT. Since the artifact has been widely observed in many silicate glasses, these results indicate that much of the existing nucleation rate data at low temperatures are incorrect.

cond-mat.mtrl-sci↗

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

Time-dependent nucleation rate measurements in BaO.2SiO2 and 5BaO.8SiO2 glasses

The two-step heat treatment method is used to measure the steady state crystal nucleation rate and induction time as a function of temperature in BaO.2SiO2 and 5BaO.8SiO2 glasses. For both glasses, the temperature for maximum nucleation rate and the temperature range for significant nucleation agree well with previous estimates from differential thermal analysis studies. The data are analyzed with a new iterative method to obtain the interfacial free energy and critical work of cluster formation as a function of temperature. For temperatures below the temperature at which steady-state nucleation rate is a maximum, the critical work of cluster formation is shown to strongly deviate from expectations of the classical theory of nucleation.

cond-mat.mtrl-sci↗

arXiv2019

Maximum Supercooling Studies in Ti39.5Zr39.5Ni21 and Zr80Pt20 - Connecting Liquid Structure and the Nucleation Barrier

Almost three quarters of a century ago, Charles Frank proposed that the deep supercooling observed in metallic liquids is due to icosahedral short-range order (ISRO), which is incompatible with the long-range order of crystal phases. Some evidence in support of this hypothesis has been published previously. However, those studies were based on a small population of maximum supercooling measurements before the onset of crystallization. Here, the results of a systematic statistical study of several hundred maximum supercooling measurements on Ti39.5Zr39.5Ni21 and Zr80Pt20 liquids are presented. Previous X-Ray and neutron scattering studies have shown that the structures of these liquid alloys contain significant amounts of ISRO. The results presented here show a small work of critical cluster formation (W* = 31 - 40 kBT) from the analysis of the supercooling data for the Ti39.5Zr39.5Ni21 liquid, which crystallizes to a metastable icosahedral quasicrystal. A much larger value (W* = 60 - 99 kBT) was obtained for the Zr80Pt20 liquid, which does not crystallize to an icosahedral quasicrystal. Taken together, these results significantly strengthen the validity of Frank's hypothesis.

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↗

arXiv2017

Critical assessment of the equilibrium melting-based, energy distribution theory of supercooled liquids and application to jammed systems

Despite decades of intense study, the mechanisms underlying the extraordinary dynamics of supercooled liquids as they approach the glass transition remain, at best, mis-characterized, and at worst, misunderstood. A long standing endeavor is to understand the remarkable increase of the viscosity with supercooling. Recently, a new theory of supercooled liquids has been proposed that starts from first principles, using elementary statistical mechanics arguments, to derive a form for the viscosity that contains only a single fitting parameter in its simplest form. In this we demonstrate that this exact same form may be derived from a different starting point, and then critically examine its performance. In the process we find that functional form proposed fits the viscosity data of a diverse group of 45 liquids exceptionally well over a wide temperature range, and uncover a number of interesting correlations of the single parameter with various thermodynamic quantities, ultimately allowing for the prediction of low temperature viscosity from high temperature data. Additionally, we find that similar physical reasoning can be used to derive a similar, single parameter form for the viscosity of hard-sphere/jammed liquids. We demonstrate that this form accurately reproduces the viscosity of hard-spheres, suggesting an underlying universality in metastable dynamics.

cond-mat.dis-nn↗

arXiv2016

Correlation between Fragility and the Arrhenius Crossover Phenomenon in Metallic, Molecular, and Network Liquids

We report the observation of a distinct correlation between the kinetic fragility index $m$ and the reduced Arrhenius crossover temperature $θ_A = T_A/T_g$ in various glass-forming liquids, identifying three distinguishable groups. In particular, for 11 glass-forming metallic liquids, we universally observe a crossover in the mean diffusion coefficient from high-temperature Arrhenius to low-temperature super-Arrhenius behavior at approximately $θ_A \approx 2$ which is in the stable liquid phases. In contrast, for fragile molecular liquids, this crossover occurs at much lower $θ_A \approx 1.4$ and usually in their supercooled states. The $θ_A$ values for strong network liquids spans a wide range higher than 2. Intriguingly, the high-temperature activation barrier $E_\infty$ is universally found to be $\sim 11\ k_B T_g$ and uncorrelated with the fragility or the reduced crossover temperature $θ_A$ for metallic and molecular liquids. These observations provide a way to estimate the low-temperature glassy characteristics ($T_g$ and $m$) from the high-temperature liquid quantities ($E_\infty$ and $θ_A$).

cond-mat.soft↗

arXiv2016

A phase space approach to supercooled liquids and a universal collapse of their viscosity

A broad fundamental understanding of the mechanisms underlying the phenomenology of supercooled liquids has remained elusive, despite decades of intense exploration. When supercooled beneath its characteristic melting temperature, a liquid sees a sharp rise in its viscosity over a narrow temperature range, eventually becoming frozen on laboratory timescales. Explaining this immense increase in viscosity is one of the principle goals of condensed matter physicists. To that end, numerous theoretical frameworks have been proposed which explain and reproduce the temperature dependence of the viscosity of supercooled liquids. Each of these frameworks appears only applicable to specific classes of glassformers and each possess a number of variable parameters. Here we describe a classical framework for explaining the dynamical behavior of supercooled liquids based on statistical mechanical considerations, and possessing only a single variable parameter. This parameter varies weakly from liquid to liquid. Furthermore, as predicted by this new classical theory and its earlier quantum counterpart, we find with the aid of a small dimensionless constant that varies in size from $\sim 0.05-0.12$, a universal (16 decade) collapse of the viscosity data as a function of temperature. The collapse appears in all known types of glass forming supercooled liquids (silicates, metallic alloys, organic systems, chalcogenide, sugars, and water).

cond-mat.soft↗

arXiv2016

Universal collapse of the viscosity of supercooled fluids

All liquids in nature can be supercooled to form a glass. Surprisingly, although this phenomenon has been employed for millennia, it still remains ill-understood. Perhaps the most puzzling feature of supercooled liquids is the dramatic increase in their viscosity as the temperature ($T$) is lowered. This precipitous rise has long posed a fundamental theoretical challenge. Numerous approaches currently attempt to explain this phenomenon. When present, data collapse points to an underlying simplicity in various branches of science. In this Letter, we report on a 16 decade data collapse of the viscosity of 45 different liquids of all known types. Specifically, the viscosity of supercooled liquids scaled by their value at their respective equilibrium melting temperature ($η(T)/η(T_{melt}))$ is, for all temperatures $T<T_{melt}$, a universal function of $(T_{melt} - T)/(B T)$ where $B$ is a constant that does not change significantly from one liquid to another. This exceptionally plain behavior hints at a link between glassy dynamics and the conventional equilibrium melting transition in all known supercooled fluids.

cond-mat.soft↗

arXiv2016

A Dramatically Growing Shear Rigidity Length Scale in a Supercooled Glass Former ($NiZr_2$)

Finding a suitably growing length scale that increases in tandem with the immense viscous slowdown of supercooled liquids is an open problem associated with the glass transition. Here, we define and demonstrate the existence of one such length scale which may be experimentally verifiable. This is the length scale over which external shear perturbations appreciably penetrate into a liquid as the glass transition is approached. We provide simulation based evidence of its existence, and its growth by at least an order of magnitude, by using molecular dynamics simulations of NiZr2, a good fragile glass former. On the probed timescale, upon approaching the glass transition temperature from above, this length scale, ξ is also shown to be consistent with Ising-like scaling. Furthermore, we demonstrate the possible scaling of ξ about the temperature at which super-Arrhenius growth of viscosity, and a marked growth of the penetration depth sets in. Our simulation results suggest that upon supercooling, marked initial increase of the shear penetration depth in fluids may occur in tandem with the breakdown of the Stokes-Einstein relation.

cond-mat.mtrl-sci↗