Electromagnetic levitation containerless processing of metallic materials in microgravity: rapid solidification.

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 by ground-based investigations, a major thrust is to develop and refine robust computational tools based on theoretical and applied approaches. This work is accomplished in conjunction with experiments designed for precise model validation with application to a broad range of industrial processes.

Quantum mechanical interpretation of the minimum viscosity of metallic liquids.

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 been paid by materials scientists and the fluid dynamics community, perhaps because of the general belief that quantum mechanics is of little consequence for classical fluid dynamics. Here, considering the extrapolated high-temperature viscosity of 32 metallic alloy liquids as representative of minimum viscosity, experimental results are presented and evaluated in terms of a number of quantum- and statistical-mechanics-based theories. The surprising result is that the experimental data are within one order of magnitude of estimates from those theories. That quantum mechanics could be of importance at such high temperatures in conventional classical fluids is quite interesting. Another surprise is that the minimum viscosities of metallic alloy liquids are not too different from an archetypal quantum liquid, such as He.

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.

Resistivity Saturation in Metallic Liquids Above a Dynamical Crossover Temperature Observed in Measurements Aboard the International Space Station.

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 Zr_{64}Ni_{36} and Cu_{50}Zr_{50} liquids above a dynamical crossover temperature for the viscosity (T_{A}). The measurements were made for the levitated liquids under the microgravity conditions of the International Space Station. Based on recent molecular dynamics simulations, the saturation is likely due to the ineffectiveness of electron-phonon scattering above T_{A} when the phonon lifetime becomes too short compared to the electron relaxation time. This is different from the conventional resistivity saturation mechanisms in solids.

Maximum supercooling studies in Ti39.5Zr39.5Ni21, Ti40Zr30Ni30, and Zr80Pt20 liquids-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 had 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, Ti40Zr30Ni30, 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* = 54-79 kBT and W* = 60-99 kBT) was obtained for the Ti40Zr30Ni30 and Zr80Pt20 liquids, respectively, which do not crystallize to an icosahedral quasicrystal. Taken together, these results significantly strengthen the validity of Frank's hypothesis.

A re-evaluation of thermal expansion measurements of metallic liquids and glasses from x-ray scattering experiments.

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 structure factor showed an expansion, but both at rates much different from those expected from the direct volume measurements. In addition, the relationship between atomic volume and the characteristic lengths obtained from the structure factor from scattering experiments was found to have a fractional exponent instead of one equal to three, as expected from the Ehrenfest relation. This has led to the speculation that the atomic packing in liquids and glasses follow a fractal behavior. These issues are revisited in this study using more in-depth analysis of recent higher resolution data and some new ideas suggested in the literature. The main conclusion is that for metallic alloys, at least to a large extent, most of these anomalies arise from complicated interplays of the temperature dependences of the various partial structure factors, which contribute to the total intensities of the scattering peaks.

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.

Anomalous thermal contraction of the first coordination shell in metallic alloy liquids.

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 number of binary, ternary, and quaternary liquids in the equilibrium and supercooled (metastable liquid below the liquidus temperature) states. The data were obtained from x-ray scattering and volume measurements on levitated liquids using the electrostatic levitation technique. Although the volumes of all liquids expand with increasing temperature, the peak positions in g(r) for the first coordination shells contract for the majority of alloy liquids studied. The second and third peaks in g(r) expand, but at rates different from those expected from the volume expansion. This behavior is explained qualitatively in terms of changes in the coordination numbers and bond-lengths as clusters in liquids break up with increasing temperature.

Volume expansion measurements in metallic liquids and their relation to fragility and glass forming ability: an energy landscape interpretation.

Recent studies of Cu-Zr glasses have reported a rapid variation in the amorphous phase density near the optimal glass forming compositions, supporting the belief that the densest liquids are also the best glass formers. Here, we show that the measured densities of the Cu-Zr liquids at higher temperatures are not peaked sharply near these compositions, but the volume expansivities are. Theoretical studies have shown that the expansivity correlates with fragility near T(g); the experimental results presented here show that at high temperature they become anticorrelated. From energy landscape arguments, this indicates the existence of a crossover temperature for the expansivity-fragility correlation that scales inversely with the liquid fragility. These results lead to an improved understanding of the high temperature properties of liquids that form glasses and suggest a new method for identifying the best glass forming compositions within an alloy system from the properties of the equilibrium liquids.

Inverse correlation between cohesive energy and thermal expansion coefficient in liquid transition metal alloys.

The volume expansion coefficients (α) of twenty-five glass-forming transition metal alloy liquids, measured using the electrostatic levitation technique, are reported. An inverse correlation between α and the cohesive energy is found. The predicted values of α from this relationship agree reasonably well with the published data for thirty other transition metal and alloy liquids; some disagreement was found for a few alloys containing significant amounts of group III and IV elements. A theoretical argument for this empirical relationship is presented.

Self-organized bimetallic Ag-Co nanoparticles with tunable localized surface plasmons showing high environmental stability and sensitivity.

We demonstrate a promising synthesis route based on pulsed laser dewetting of bilayer films (Ag and Co) to make bimetallic nanoparticle arrays. By combining experiment and theory we establish a parameter space for the independent control of composition and diameter for the bimetallic nanoparticles. As a result, physical properties, such as the localized surface plasmon resonance (LSPR), that depend on particle size and composition can be readily tuned over a wavelength range one order of magnitude greater than for pure Ag nanoparticles. The LSPR detection sensitivity of the bimetallic nanoparticles with narrow size distribution was found to be high-comparable with pure Ag (∼60 nm/RIU). Moreover, they showed significantly higher long-term environmental stability over pure Ag.

Magnetic properties of S = 1/2 quasi-triangular lattice materials: Cu(2(1-x))Zn(2x)(OH)3NO3/(C7H15COO)·mH2O.

We have investigated the structural and magnetic properties of two classes of spin S = 1/2 antiferromagnetic quasi-triangular lattice materials: Cu(2(1-x))Zn(2x)(OH)(3)NO(3) (0 ≤ x ≤ 0.65) and its long chain organic derivatives Cu(2(1-x))Zn(2x)(OH)(3)(C(7)H(15)COO)·mH(2)O (0 ≤ x ≤ 0.29). The series of layered structure compounds constitute a substitutional magnetic system, in which spin S = 1/2Cu(2+) ions and nonmagnetic Zn(2+) ions are arranged on a two-dimensional quasi-triangular lattice. For the nitrate compounds we found that the substitution of Zn(2+) ions can continuously decrease the Néel temperature, T(N), but never completely remove the magnetic order. In addition, the frustration effect in these materials is suppressed by a three-dimensional interlayer interaction. On the other hand, the corresponding long chain alkyl carboxylic acid group of intercalated materials, Cu(2(1-x))Zn(2x)(OH)(3)(C(7)H(15)COO)·mH(2)O, show spin-glass-like behavior, which is caused by the interplay of geometric frustration and mixed sign interactions. A tentative explanation for these findings is proposed in terms of a cluster-glass picture.

Icosahedral order, frustration, and the glass transition: evidence from time-dependent nucleation and supercooled liquid structure studies.

One explanation for the glass transition is a geometrical frustration owing to the development of non-space-filling short-range order (icosahedral, tetrahedral). However, experimental demonstrations of this are lacking. Here, the first quantitative measurements of the time-dependent nucleation rate in a Zr59Ti3Cu20Ni8Al10 bulk metallic glass are combined with the first measurements of the evolution of the supercooled liquid structure to near the glass transition temperature to provide strong support for an icosahedral-order-based frustration model for the glass transition in Zr-based glasses.

Pressure-induced superconductivity in CaLi(2).

A search for superconductivity has been carried out on the hexagonal polymorph of Laves-phase CaLi(2), a compound for which Feng, Ashcroft, and Hoffmann predict highly anomalous behavior under pressure. No superconductivity is observed above 1.10 K at ambient pressure. However, high-pressure ac susceptibility and electrical resistivity studies to 81 GPa reveal bulk superconductivity in CaLi(2) at temperatures as high as 13 K. The normal-state resistivity displays a dramatic increase with pressure.

Containerless measurements of thermophysical properties of Zr54Ti8Cu20Al10Ni8.

High-temperature measurement and study of reactive materials can be difficult with conventional processing methods because contamination from the measuring apparatus and container walls can adversely affect measurements. Containerless processing techniques can be employed to isolate samples from their environment, reducing contamination. Benefits of containerless processing include reduction in heterogeneous nucleation sites, which in turn delays the onset of solidification and allows the study of meta-stable undercooled phases. However, property measurements must use noncontact methods as well. Fortunately, several optical-based methods have been developed and successfully employed to measure thermophysical properties, including surface tension, viscosity, density, and thermal expansion. Combining these techniques with the electrostatic levitator (ESL) located at the NASA Marshall Space Flight Center (MSFC) has resulted in an excellent facility to perform containerless material studies which support microgravity flight projects. Currently, studies of the thermophysical properties of liquid quasi-crystal forming and related alloys ranging from superheated to deeply undercooled states are being done with this facility in support of the NASA-funded flight project Quasi-crystalline Undercooled Alloys for Space Investigation (QUASI). While the primary purpose of these measurements is to support planned flight experiments, they are also a desirable resource for future manufacturing considerations and for fundamental insight in the physics of icosahedral ordering in liquids and solids. Presented here is an overview of the contactless measuring methods for surface tension, viscosity, density, and thermal expansion applied to Zr54Ti8Cu20Al10Ni8, for the superheated and meta-stable undercooled liquid phases, in support of QUASI.

In situ high-energy x-ray diffraction study of the local structure of supercooled liquid Si.

Employing the technique of electrostatic levitation, coupled with high-energy x-ray diffraction and rapid data acquisition methods, we have obtained high quality structural data more deeply into the supercooled regime of liquid silicon than has been possible before. No change in coordination number is observed in this temperature region, calling into question previous experimental claims of structural evidence for the existence of a liquid-liquid phase transition.

Difference in icosahedral short-range order in early and late transition metal liquids.

New short-range order data are presented for equilibrium and undercooled liquids of Ti and Ni. These were obtained from in situ synchrotron x-ray diffraction measurements of electrostatically levitated droplets. While the short-range order of liquid Ni is icosahedral, consistent with Frank's hypothesis, significantly distorted icosahedral order is observed in liquid Ti. This is the first experimental observation of distorted icosahedral short-range order in any liquid, although this has been predicted by theoretical studies on atomic clusters.

First x-ray scattering studies on electrostatically levitated metallic liquids: demonstrated influence of local icosahedral order on the nucleation barrier.

To explain the unusual stability of undercooled liquids against crystallization, Frank hypothesized that the local structures of undercooled liquids contain a significant degree of icosahedral short-range order, which is incompatible with long-range periodicity. We present here the first direct experimental demonstration of Frank's complete hypothesis, showing a correlation between the nucleation barrier and a growing icosahedral short-range order with decreasing temperature in a Ti39.5Zr39.5Ni21 liquid. A new experimental facility, BESL (Beamline Electrostatic Levitation), was developed to enable the synchrotron x-ray structural studies on deeply undercooled, reactive liquids.

Bacillary angiomatosis in an immune-competent patient.

Bacillary angiomatosis is reported in a 65-year-old man. He was immunocompetent and the lesions subsided following treatment with oral erythromycin.

Ichthyosiform sarcoidosis.

A rare case of ichthyosiforn sarcoidosis is being reported.