K. F. Kelton

Washington University in St. Louis | WUSTL , Wash U · Department of Physics

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 d...

Space levitation processing allows researchers to conduct benchmark tests in an effort to understand the physical phenomena involved in rapid solidification processing, including alloy thermodynamics, nucleation and growth, heat and mass transfer, solid/ liquid interface dynamics, macro-and microstructural evolution, and defect formation. Supported...

Metallic glasses have the potential to become transformative materials, but this is hindered by the lack of ability to accurately predict which metallic alloys will form good glasses. Current approaches are limited to empirical rules that often rely on parameters that are unknown until the glasses are made, rendering them not predictive. In this Pe...

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 d...

Possible fundamental quantum bounds for viscosity and many other physical properties have drawn serious considerations recently from diverse communities encompassing those studying quantum gravity, high-energy physics, condensed matter physics, strongly correlated electron systems, and “strange metals,” to name a few. However, little attention has...

The results of a combined experimental and computational investigation of the structural evolution of Au 81 Si 19 , Pd 82 Si 18 , and Pd 77 Cu 6 Si 17 metallic glass forming liquids are presented. Electrostatically levitated metallic liquids are prepared, and synchrotron x-ray scattering studies are combined with embedded atom method molecular dyna...

Liquids realize a highly complex state of matter in which strong competing kinetic and interaction effects come to life. As such, liquids are, generally, more challenging to understand than either gases or solids. In weakly interacting gases, the kinetic effects dominate. By contrast, low temperature solids typically feature far smaller fluctuation...

Nucleation is the first step in most first-order phase transitions, which include processes such as gas condensation, solidification, and the fabrication of glass ceramics. It also plays a key role in some biological processes and is fundamentally important in the pharmaceutical industry. Nucleation is commonly described within the framework of the...

In addition to its fundamental importance, the electrical resistivity plays a very important role in the studies of various types of thermal excitations (phonons, excitons, magnons) and phase transitions (structural, magnetic, electronic, superconducting) in crystalline solids. It is also useful in amorphous materials for studying the glass transit...

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...

The results of a combined structural and dynamical study of Cu–Zr–Al metallic glass forming liquids are presented. Containerless high-energy x-ray scattering experiments made using electrostatic levitation are combined with molecular dynamics simulations to probe the onset of rapid structural ordering as well as the temperature-dependent diffusivit...

Liquids realize a highly complex state of matter in which strong competing kinetic and interaction effects come to life. As such, liquids are, generally, more challenging to understand than either gases or solids. In weakly interacting gases, the kinetic effects dominate. By contrast, low temperature solids typically feature far smaller fluctuation...

For over 40 years, measurements of the nucleation rates in a large number of silicate glasses have indicated a breakdown in the Classical Nucleation Theory at 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...

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...

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 t...

The evaluation of the crystal growth in undercooled alloy melts is essential for investigations of their solidification behavior. Low-melting alloy systems usually exhibit little self-illumination and cinematography is not often feasible. A time-temperature data based statistical learning approach is presented to evaluate the crystal growth in an u...

The accuracy of a Differential Thermal Analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a 5BaO·8SiO2 glass based on iterative numerical calculations. The diffusion coefficient is calculated from the measured induction time and the crystal growth velocity. The Classical Theory of Nucleation and t...

The specific volumes and thermal expansion coefficients of 41 transition-metal based alloy liquids, which include both bulk and marginal glass-formers, are presented. Those parameters are compared with their values either in the corresponding crystal phases or in their constituent elemental liquids. The volume differences in both cases at the liqui...

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 t...

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 formati...

The manuscript titled “Modeling nonisothermal crystallization in a BaO∙2SiO2 glass” contains two minor mistakes. This errata acknowledges and corrects the errors. The corrections clarify the details of the simulations performed in the original work and do not impact the results.

The structural evolution of the equilibrium and supercooled Cu46Zr54 liquids was investigated with a combination of elastic neutron scattering (with isotopic substitution) and synchrotron x-ray scattering studies. The partial pair correlation functions were determined over a wide temperature range (∼270 °C). These show that the Cu–Cu and Zr–Zr orde...

During containerless processing, the oscillating drop method can be used to measure the surface tension and viscosity of a levitated melt. Through containerless processing, reactive melts that cannot be measured through conventional methods can be accurately measured; however, the accuracy of this method is dependent on the internal flow within the...

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 Z...

We present the results of a structural study of metallic alloy liquids from high temperature through the glass transition. We use high energy X-ray scattering and electro-static levitation in combination with molecular dynamics simulation and show that the height of the first peak of the structure function, S(Q) − 1, follows the Curie-Weiss law. Th...

Glass formation is an interesting phenomenon in condensed matter. Many studies have shown that the reduced glass transition temperature, Tg/Tl (where Tg and Tl are the glass transition and liquidus temperature), and the liquid fragility, m, play an important role in metallic glass formation. The combination of these parameters can lead to a predict...

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 th...

The accuracy of a differential thermal analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a BaO∙2SiO2 glass by iterative numerical calculations. The numerical model takes account of time‐dependent nucleation, finite particle size, size‐dependent crystal growth rates, and surface crystallization. T...

Although a resistivity saturation (minimum conductivity) is often observed in disordered metallic solids, such phenomena in the corresponding liquids are not known. Here we report a saturation of the electrical resistivity in Zr64Ni36 and Cu50Zr50 liquids above a dynamical crossover temperature for the viscosity (TA). The measurements were made for...

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 had been published previously. However, those studies were based on a sm...

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 th...

The surface tension and viscosity of equilibrium and supercooled liquids of Cu50Zr50 were measured in the containerless electromagnetic levitator ISS-EML in the European space laboratory Columbus on board the International Space Station (ISS) under microgravity using high-speed camera recordings. From 1250 K to 1475 K, the surface tension follows t...

The glass transition temperature, T g , is important for predicting glass formation and stability and for designing processing steps for tailoring the glass to specific applications. It is conventionally determined from measurements of the shear viscosity or specific heat. This requires that the glass first be made, significantly limiting the use o...

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 sm...

Expressions for X-ray absorption and secondary scattering are developed for cylindrical sample geometries. The incident-beam size is assumed to be smaller than the sample and in general directed off-axis onto the cylindrical sample. It is shown that an offset beam has a non-negligible effect on both the absorption and multiple scattering terms, res...

Since extremely high quench rates (10 ¹² to 10 ¹⁴ K/s) are normally necessary to form elemental metallic glasses, very few are known. Even when synthesized, very little is known about their properties because of the limited quantities of material produced under such extreme conditions. In particular, the glass transition temperature, T g , of most...

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. M...

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...

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...

Even though viscosity is one of the most fundamental properties of liquids, determining its atomistic origin has proven elusive. By combining inelastic neutron scattering with the electro-static levitation technique the time-dependent pair-distribution function (i.e. the Van Hove function) has been determined for liquid $\text{Zr}_{80} \text{Pt}_{2...

A revised version of Table 2 of Bendert et al. [Acta Cryst. (2013). A69, 131–139] is provided.

It is widely, although not universally, believed that there must be a connection between liquid dynamics and the structure. Previous supporting studies, for example, have demonstrated a link between the structural evolution in the liquid and kinetic fragility. Here, new results are presented that strengthen the evidence for a connection. By combini...

Previous studies reported a number of anomalies when estimates of linear thermal expansion coefficients of metallic liquids and glasses from x-ray scattering experiments were compared with direct measurements of volume/length changes with temperature. In most cases, the first peak of the pair correlation function showed a contraction, while the str...

The dynamical behaviour of liquids is frequently characterized by the fragility, which can be defined from the temperature dependence of the shear viscosity, η (ref. ). For a strong liquid, the activation energy for η changes little with cooling towards the glass transition temperature, Tg. The change is much greater in fragile liquids, with the ac...

The advent of containerless processing techniques has opened the possibility of high quality measurements of equilibrium and metastable liquids. This review focuses on the structure and dynamics of metallic liquids at high temperature. A clear connection between structure, viscosity, and fragility has emerged from recent containerless experiments a...

We report on trends observed in the X-ray static structure factor obtained for four homologous imidazolium-based ionic liquids. In a series of low-energy (laboratory) and high-energy synchrotron X-ray diffraction experiments, the temperature dependent structure was investigated in 1-alkyl-3-methylimidazolium cations, with chain lengths of n = 4, 6,...

From measurements of X-ray and neutron scattering of electrostatically levitated Zr77Rh23 liquids, a variety of metastable crystallization behavior was observed. The metastable phase selection in deeply undercooled liquid droplets is characterized and their crystallization pathways discussed. A metastable phase previously identified as a primary de...

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 an...

Structural data from the supercooled Zr55Cu35Al10 ternary alloy liquid were collected in-situ in the temperature range from 1172 K to 953 K during continuous cooling using the high-energy synchrotron X-ray Advanced Photon Source. The lowest-r peaks of the pair distribution functions (PDFs) sharpened, becoming higher and narrower, with decreasing te...

A differential thermal analysis (DTA) based method is used to estimate the temperature range for crystal nucleation in BaO·2SiO2 and 5BaO·8SiO2 glasses. For the BaO·2SiO2 glass, the temperature range for significant nucleation is approximately 660 °C to 770 °C, with a maximum rate near 712 °C. These results are in good agreement with those determin...

We study the relation of crystal-liquid interfacial free energy and medium range order in the quasicrystal-forming Ti37Zr42Ni21 liquid from undercooling experiment and ab initio molecular dynamics (MD) simulation. Adding a small amount of Ag to the liquid significantly reduces the degree of undercooling, which is suggestive of small interfacial fre...

The dynamical behavior of liquids is frequently characterized by the fragility, which can be defined from the temperature dependence of the shear viscosity, {\eta}. For a strong liquid, the activation energy for {\eta} changes little with cooling towards the glass transition temperature, Tg. The change is much greater in fragile liquids, with the a...

Many phenomena in the world around us depend on infrequent, yet short-lived, events that completely alter how a system subsequently develops in time. In the physical sciences, there are many examples of such crucial “rare events.” Among the most important of these are nucleation processes, in which, due to a rare fluctuation, a new phase forms spon...

The state-of-the-art experimental and atomistic simulation techniques were utilized to study the structure of the liquid and amorphous Ni 62 Nb 38 alloy. First, the ab initio molecular dynamics (AIMD) simulation was performed at rather high temperature where the time limitations of the AIMD do not prevent to reach the equilibrium liquid structure....

The liquid phase remains poorly understood. In many cases, the densities of liquids and their crystallized solid phases are similar, but since they are amorphous they lack the spatial order of the solid. Their dynamical properties change remarkably over a very small temperature range. At high temperatures, near their melting temperature, liquids fl...

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 t...

Among the three fundamental processes of heat transfer (conduction, convection, and radiation), radiation is the most dominant at high temperatures. The total hemispherical emissivity is an important property that determines the amount of heat loss by radiation. Unfortunately, the emissivity, especially its temperature dependence \((\varepsilon (T)...

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 fundamenta...

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 perturbat...

Time-resolved synchrotron measurements were carried out to capture the structure evolution of an electrostatically levitated metallic-glass-forming liquid during free cooling. The experimental data shows a crossover in the liquid structure at ∼1000 K, about 115 K below the melting temperature and 150 K above the crystallization temperature. The st...

We report the observation of a distinct correlation between the kinetic fragility index $m$ and the reduced Arrhenius crossover temperature $\theta_A = T_A/T_g$ 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 co...

Recent experimental results suggest that metallic liquids universally exhibit a high-temperature dynamical crossover, which is correlated with the glass transition temperature (). We demonstrate, using molecular dynamics results for , that this temperature, , is linked with cooperative atomic rearrangements that produce domains of connected icosahe...

The short-range order (SRO) and medium-range order of electrostatically levitated Zr80Pt20 and Zr77Rh23 liquids are presented based on a combination of high-energy x-ray diffraction and time-of-flight neutron diffraction studies. The atomic structures of the Zr80Pt20 liquids were determined as a function of temperature from constrained reverse Mont...

Neutron diffraction studies of metallic liquids provide valuable information about inherent topological and chemical ordering on multiple length scales as well as insight into dynamical processes at the level of a few atoms. However, there exist very few facilities in the world that allow such studies to be made of reactive metallic liquids in a co...

Despite decades of intense study, the underlying mechanism of the extraordinary dynamics of liquids approaching the glass transition remains, at best, mischaracterized, and at worst, misunderstood. With sufficiently rapid cooling, any liquid can, in principle, be made to form a glass, displaying an immense increase in viscosity upon supercooling. A...

The existence of a 'crossover region' in glass-forming liquids has long been considered as a general phenomenon that is as important as the glass transition. One potential origin for the crossover behavior is a liquid-to-liquid phase transition (LLPT). Although a LLPT is thought to exist in all forms of liquids, structural evidence for this, partic...

The range of magnitude of the liquid viscosity, η, as a function of temperature is one of the most impressive of any physical property, changing by approximately 17 orders of magnitude from its extrapolated value at infinite temperature (ηo) to that at the glass transition temperature, Tg. We present experimental measurements of containerlessly pro...

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 perturbat...

Thermal evaporation loss measurements made using the electrostatic levitation (ESL) technique for one binary Ti-Zr, two ternary Ti-Zr-Ni, and two glass-forming (Vit 106 and Vit 106a) alloy liquids are reported. The containerless environment enables measurements not only for the equilibrium liquids but also for the metastable supercooled liquids. Th...

While X-ray scattering studies can provide some insight into the average topological structures of liquids, they provide limited information on chemical ordering. Combined X-ray and neutron scattering studies can address this limitation, but are often difficult, time consuming and expensive, if isotopes are used for the neutron scattering measureme...

The results of high-energy X-ray scattering studies over a wide temperature range for equilibrium and supercooled ZrxNi100 − x (x = 36, 57, 76) liquids and the corresponding glasses are presented. The results of liquid density and viscosity measurements are also shown. All of the liquid studies were made on containerlessly processed liquids using t...

The densities of solid and liquid CuZr and the viscosity of the liquid were measured in a containerless electrostatic levitation system using optical techniques. The measured density of the liquid at the liquidus temperature (1223 K) is (7.02 0.01) g cm and the density of the solid extrapolated to that temperature is (7.15 0.01) g cm. The thermal e...

Accurate and precise measurements of the temperature are important for controlling conditions in any experimental design. In containerless processing, such as in electrostatic levitation and electromagnetic levitation, the temperature is typically measured using optical pyrometers. These are generally calibrated to the sample of interest by measuri...

Quantum effects in material systems are often pronounced at low energies and become insignificant at high temperatures. As we elaborate here, this common occurrence might not, however, hold universally. We find that, perhaps counterintuitively, certain quantum effects may follow the opposite route and become progressively sharper so as to emerge in...

Virtually all liquids can be maintained for some time in a supercooled state, that is, at temperatures below their equilibrium melting temperatures, before eventually crystallizing. If cooled sufficiently quickly, some of these liquids will solidify into an amorphous solid, upon passing their glass transition temperature. Studies of these supercool...

The range of the magnitude of the liquid viscosity as a function of the temperature (T) is one of the most impressive of any physical property, changing by approximately 17 orders of magnitude from its extrapolated value at infinite temperature to that at the glass transition. We present experimental measurements of containerlessly processed metall...

Measurements of sharp diffraction peaks as a function of temperature are routinely used to obtain precise linear expansion coefficients of crystalline solids. In this case, the relation between temperature dependent changes in peak position in momentum transfer (q1) and volume expansion is straightforward (Ehrenfest's relation: q1 = K(2π/d), where...

Except for a few anomalous solids and liquids, materials expand upon heating. For liquids, this should be reflected as a shift in the peak positions in the pair correlation function, g(r), to higher r. Here, we present the results of a detailed study of the volume thermal expansion coefficients and the temperature dependences of g(r) for a large nu...

Quasicrystals may have important applications as new technological materials. In particular, work in our laboratory has shown that some quasicrystals may be useful as hydrogen-storage materials. Some transition metals have a capacity to store hydrogen to a density exceeding that of liquid hydrogen. Such systems allow for basic investigations of sol...

The results of high energy X-ray scattering studies of equilibrium and supercooled Cu100-xZrx (x = 46 and 54) and CuxHf100-x (x = 55 and 60.8) liquids and the corresponding glasses are presented. The liquid data were obtained in a containerless environment using the beamline electrostatic levitation (BESL) technique. The total structure factor and...

A systematic study of the viscosity in the equilibrium and supercooled states is reported for Cu-100 - Zr-x(x) (30 <= x <= 55) liquids. While no correlation between the magnitude of the viscosity at T = T-g/0.6 with the glass forming ability is observed, peaks in the kinetic strength are observed at the best glass forming compositions. This not onl...